Linda Whalley is watching a video of her old self with a protective hand in front of her eyes. She can hardly bear to look. "Nobody's ever seen this before," she says. She wobbles around the garden, breathless and sweaty, ankles swollen, body hidden in a huge tent dress, and she's telling the camera how well she's done. "This is Linda Whalley, I've lost seven stone and it's June 21 2002." At the time she weighed 23 stone (146kg).

Whalley recorded herself as she lost weight. The videos are incredibly moving – like watching somebody slowly come back to life. "I've lost 10 stone now, and I feel absolutely wonderful. This is the new me," she says walking round her garden with relative ease.

A few months later she tells the camera: "What used to be a size 40 smock dress is now size 22. I can wear proper shoes, not sandals with my feet bulging all over the top, and I can walk for miles. The only thing I haven't done yet is swim. I feel I'm still too big with all my wobbly bits that I wouldn't want anyone to see."

"I have now lost 13 stone two-and-a-half pounds and I feel absolutely fabulous. Today I'm going to the wedding of my best friend's daughter, and I'm going to see a lot of people I've not seen since before I started slimming, so I think it will be a bit of a shock for one or two of them. I think that's it for today. Byeee." She gives her signature wave to the camera.

By the time she won Slimming World's Greatest Loser award, she had lost 18 of her 30st. As we watch the video, she smiles with pride and embarrassment. "Why do I always do that silly wave at the end? Anyway, that was me."

It's a lovely summer's day in Stoke, and we retire to her beautifully tended garden. Linda's husband, Charlie, cracks open a couple of beers for him and me, she sticks to water.

Linda, 59, knows if she'd carried on the way she had been, she might well have been dead by now. But the nearest she got to a warning at the time was a GP's suggestion that she eat off a saucer rather than a dinner plate.

She was aware of her weight from her earliest school days. Her uniform was different from her fellow pupils – it was adult size. She remembers the great big collars and sleeves and how self-conscious they made her feel, but she had friends, she could still play sport, was still one of the girls.

Then one day the school nurse sent home a letter for her mother saying she had to go on a diet. "It was Friday and Friday was always chippy day. Dad got paid and we'd have chips, steak pudding, mushy peas and gravy. And this day I came home and I'd got chips, pudding, gravy and, because I was on a diet, we didn't have the peas."

The diets didn't work, and she grew into an even bigger girl. It didn't put Charlie off when they got together. She was 17 and size 18 – a big, pretty young thing, and he thought she was the best. "It's the personality, isn't it?" he says.

By the age of 21, when they married, she had reached 18st and decided to diet again. She lost eight and a half stone on what was little more than a glorified starvation regime that left her bad tempered and constantly hungry.

"I was evil sometimes. When you're hungry you try to make an argument about anything, don't you?" She'd go to bed early to avoid eating.

The thing is, Whalley says, she lived for food. And she had always been an emotional eater. If she was happy, she'd eat; if she was sad, she'd eat. "I get pleasure out of talking about food, buying it, eating it, thinking about it. I know people don't think there's an addiction, as in smoking, drink and drugs, but I do." What's her favourite food? "I like everything. Chocolate, biscuits, everything. And if there was 50% extra, I'd eat that as well, because that's free, isn't it? I hadn't paid for it, so it didn't matter."

She started working nights and put on more weight because she was pretty much eating around the clock. Everybody knew she took in huge bags of fruit to eat on her shift. What was less known was that she also took in huge bags of chocolate. "Looking back now, I bet people said, 'I'm not going to eat that much bloody fruit if you end up like Linda.' " Whalley never drank or smoked. She never had the time – she was too busy eating.

Although she was becoming less and less mobile, so much in her life was good – she had a loving husband, a great job with a computer manufacturer, friends. And she always made the best of herself. "If you're as big as I was, there's a stigma – that you're dirty and smelly and bone idle. I've always had my hair done and worn lippy, always treated myself to perfumes, because I couldn't get the clothes." On the rare occasions people were rude about her weight, Charlie would defend her honour. "One day, I had a new dress on, and I was coming out of a shop when I got a tap on the shoulder and some lady said…"

Charlie frowns: "No, she wasn't a lady."

"She goes, 'Excuse me, have you got an illness that you're so fat?' And Charlie said, 'Have you got an illness that you're so rude?' and that was it. We had to come home. I was crying, but I still didn't do anything about it."

Some men might have been embarrassed about having such a large wife, I say to Charlie. His voice rises slightly when he answers. "No, because I love her."

Things were bound to come to a head, and sure enough they did. After 27 years, she lost her job. At the weight she was, she knew it would be almost impossible to find another. She managed the job she was doing only because they allowed her to park so near her office that she barely had to walk. As for working in a shop, who was going to employ somebody who looked like her? The day she was made redundant she just sat and ate Penguin bar after Penguin bar – 30 in all.

Did she feel sick? "Not physically, mentally I did. I also felt pain in here." She points to her heart. "And I started to cry. I thought, 'This is what I'm going to do every day – just eat.' And I didn't want to." She was feeling so hopeless about her future that she was frightened to tell Charlie. It would have been so easy for her to go under. Instead, she acted on an advert for Slimming World that promised you could lose weight by eating loads – so long as you ate the right stuff.

Whalley dreaded the weigh-in. It was a while since she'd been on the scales – and she was sure she was more than 25st. In fact, she was 30st 9lb.

The diet was perfect for her – some foods were free ("I could eat washing-up bowls full of cereal or pasta") and some – chocolate, crisps, cakes and alcohol – were controlled. She knew if she didn't make a go of this, she was running out of options.

In her lounge, there is a life-size cardboard cutout of Big Linda, which she was presented with when she won Greatest Loser in 2005. "Isn't that the worst title ever?" She laughs. She won £3,000 and a night at the Ritz for her and Charlie. "Then there are all the lovely things that come afterwards – you go to the  Slimming World ball and all the celebrities are there – Bradley Walsh and Lenny Henry."

The best thing, she says, is at 12st 8lb (6lb heavier than when she won) she can live a full life: fly in planes, travel in cars without worrying about seat belts, swim, walk anywhere she fancies. "Today I've done a good two-hour walk because I couldn't go swimming, and I love it. It's the freedom. Thinking about food as I'm walking. Always thinking about food, what can we eat tonight, something we've not done for a while."

Is she surprised by what she looks like today? Whalley stutters, and admits she's not sure what she does look like. "If you saw me walking down the street, would you say, 'God, she's fat'?" Her question, and the intensity with which she asks it, surprises me. Not at all, I say. "Honestly? But I still think, 'Do I look normal?' Because that's what I want to be – normal."

"It does my bloody head in," Charlie says gently.

Is there anything she doesn't like about her new body? Plenty, she says. There's her chest, for starters. "I haven't got one, have I? It was massive." Then there's the skin round her belly. She's phlegmatic about that, "You can tuck it in your knickers, can't you?" But it's her upper arms she most dislikes. "When I was big, I wasn't embarrassed to wear nothing on my shoulders – because my arms were fat, they weren't floppy. But now I tend to cover up. I hate my arms. They're all flabby, aren't they?"

Whalley, now a consultant for Slimming World, is determined to keep off the weight, but compares her situation to that of an alcoholic – always recovering, never former. An estimated 95% of slimmers put the weight back on. It could be argued this is what sustains a £2.6bn industry.

After Susie Flack lost 10st she won Rosemary Conley Diet & Fitness Magazine's Slimmer of the Year in 2002. She was 8st 7lb (55kg), had shrunk from a size 24 to 12, and one of her highlights was being stopped by an autograph hunter who mistook her for the actor Martine McCutcheon. Today, she's back up to 10st and says she'd prefer not to be photographed. "I'm only 5ft 2in and Rosemary [Conley] saw me like this and said, 'To be honest, you look better with a couple of stone off.' She said it nicely, she was just giving advice."

For a long time, Flack says, she kept the weight off by running 13-20 miles a day. Jesus, I say. "Yes, I was running like crazy. I loved it, but it probably was a bit over the top."

Flack has always had ambitions to make it as a singer, and the slimming award gave her experience of life in the limelight. But when she went on television, she discovered there's thin and thin. "I was on with Melinda Messenger and you just looked at her shoulders and legs and realised how tiny she is. So you try to go down to seven and a half, and then telly puts on half a stone, so there's the pressure to get down to seven."

After her daughter Amelia was born, she found it hard to lose the weight. That was tough enough, but just as bad was the delight others took in it. "People are so pleased if they see you've put a bit on. They say things like, 'Oh, you do look better bigger, you've got such a pretty face', and it's always the thin girls who say that, because they see you as competition."

An estimated one in four adults are obese. The NHS spends about £7bn a year treating obesity-related health problems such as diabetes, heart disease and high blood pressure – anti-obesity prescriptions cost the NHS £47m a year. Meanwhile, there are more than 35,000 dieting books, from the established high-fibre F-plan and the low-carb Atkins, to the gourmet cabbage soup, eskimo, astrology, orgasm and pray yourself thin diets. WeightWatchers boast that 10 million Brits have walked through their doors over the past decade and Slimming World claims that, on average, 300,000 people in the UK attend their weekly meetings.

When snooker player Les Dodd lost 7st 7lb (50kg) to win WeightWatcher of the Year in 1987 he became known as Less Dodd. Unfortunately, he then put on 13 to take him to 25st (159kg). He picks me up at Southport station in his van and hobbles out to greet me. We meet a few days before he is due to have a hip replacement, and he is in agony.

Dodd is a young-looking 56 and has three grown-up children. He twice played in the snooker world championships and peaked at 38 in the world, in 1983. He was 33 when he won WeightWatcher of the Year, weighed 12st 10lb, and felt wonderful. "I had a 34-inch waist, 38-inch chest, I could do 100 push-ups, 100 back lifts." How did he lose the weight? "I followed the WeightWatchers diet – I used to eat as much as a sparrow and run and exercise." He won £100 of vouchers and a weekend in Malta. The problem is, he says, once you've lost all the weight, you feel you've reached your goal. "I didn't realise you had to be 10 times more careful. You get complacent, don't you? You've got nice clothes, you look good, you go out and have a bit to eat."

Why couldn't he keep the weight off? "Because you're a pig, aren't you? If you like your grub, you like your grub, don't you?"

After he won, he did a number of publicity events for WeightWatchers. He was the ideal ambassador – slim, semi-famous and likable. But before long he was back on the fatty foods. KFC was his favourite. "Absolute sweetheart. Family bucket and six chips to yourself. Then you want the apple pie."

Did he give up on the exercise regime? He grins. "My biggest exercise was lifting that family bucket from the Kentucky." What did he typically eat in a day? "Andy's cafe in the morning, two sausage and bacon toasties, that's 7am, get to 10am and nip into the pie shop, meat-and-potato pie, steak-and-kidney pie followed by a few custard tarts, then dinner time – when you were hungry, it was fish, chips, peas and gravy, then you'd go home and your wife would cook you a lovely tea, and then you'd be out again at night taxi-ing or playing snooker – couple of packets of crisps, then 1am, I'm hungry again, chips, sausage, peas and gravy. Honestly, it's embarrassing."

Like Whalley, eating took up so much of his life that he never had time to develop other bad habits. And like her, Dodd suffered a form of body dysmorphia when he lost weight. The new Dodd couldn't win a match to save his life, so he called in snooker legend Ray Reardon for advice. "I said to him, 'I can't understand it, I'm playing like a nightmare', and he looked at me and said, 'Christ, the table's here, and you're there – get nearer, change your stance because you've lost all your body weight.' I was still stood how I used to stand for 20 years, then I started to play well."

Does he think his weight hampered his career? "Yes. Take me out in trackie bottoms and a loose top, no problem. Put that evening suit on, then it's a different thing. You're choked up."

His fellow snooker professionals would rib him about his weight. He shows me a photograph of an obese naked woman that former snooker world champion John Spencer pinned up at one tournament, saying it was Les. "Something like that you can handle, it's a great laugh." But there were other times that weren't so funny. "About three years ago, my son Anthony was working at a roofing company and one of the guys said, 'Oh your dad's that big fat fucker', and Anthony was upset. He told me he wasn't speaking to this fella and I asked why and he said it didn't matter. Eventually, my missus told me. Comments like that are very hurtful, made behind your back."

Last year, he was made to realise that he had to lose weight – that it really was a matter of life or death. He was 25st with a 54in waist, and his doctor told him he was a heart attack waiting to happen. "It terrified me. You're going up the stairs and it's killing you, you're falling asleep as soon as you get home, you're carrying too much weight and you can't take it."

This time he decided to do it himself, and not through extreme dieting, just by eating sensibly. He's down to around 18st (114kg) and is aiming to lose another six. If he meets his target, does he think he will keep it off this time? "Definitely." What's the difference now? "I'm not going to die, simple as that. Or become paralysed and be pushed round in a wheelchair. I've got lots of lovely friends, great business, wife, three lovely kids, grandchild, what would I want to ruin all that for?"

Melanie Slinger is beautifully dressed with long pearl earrings and expensively highlighted hair. She won Slimming World Slimmer of the Year in 2002 after losing 10st. But she ended up losing more than she bargained for – her husband, her children for a year, her house, her job and her money.

We meet in Newcastle with her partner Mick, who has a big belly and a kind face. As she drinks a cappuccino, she describes how she was teased at school and made to feel ugly. She does not try to hide the pleasure she takes from the fact that, at 44, she is more glamorous than most of the girls who mocked her. "I've seen people who I went to school with who were pretty, and they're fat and ugly now. They don't take care of themselves."

In her 20s, Slinger ballooned to a size 26. She was studying for a degree, and things reached a low at 19st 4lb (123kg). "Every day I'd start a new diet, not eat till 2pm and then have six rounds of toast with two tins of beans." That's not so bad, is it? She smiles sheepishly. "But then I liked cheese on them. And after that I'd have a packet of chocolate biscuits – you just sit at the computer and pick and pick and pick."

She would turn up in the playground, and kids would taunt her children that their mum was Mrs Blobby. "They loved me, but they were embarrassed." Slinger says the idea that fat people are always happy is a myth: "'I'm happy to be big' – well, bollocks to that. You're not happy to be big at all, but nobody wants to say that because someone will turn round and say, 'Well, why don't you do something about it then?' I was the life and soul, but it's a happiness you put on." Actually, she says, in private she had been bitter and spent much of the time shouting at her children.

As with Dodd, it was a doctor's warning that made her change. "He says, 'You're borderline diabetic,' and I got a real blasting from him. He told me I could lose my legs, my arms, my eyesight, scared me shitless. And he said if you're diabetic you're going to have to start injecting yourself. Well, I'm scared of needles."

The weight peeled off quickly, and her sense of self was transformed. "I got wolf-whistled, and that was wonderful. I was going past some site with men up on the scaffolding and they started whistling. Then they shouted down, 'Hello, gorgeous.' I was looking round to see who they were talking to."

That was when she decided to enter Slimming World's Slimmer of the Year. She won a holiday to Australia, but by the time it came to taking the trip her marriage had disintegrated and her husband went with her son. "I changed completely. I was 36 years old, I suddenly had this figure, I had no idea how to handle myself. I thought I wanted to live it up. I'd never been one for going out, and felt a big chunk of my life had been missing because of my weight. Well, I wanted to recapture it. I could suddenly wear all the things I'd never been able to, like boots, trousers, nice knickers and matching bras, and I just bought and bought and bought and bought. Anything I tried on, if it fit, I bought it. I got us into a lot of debt."

How much? "At least £10,000. It was all on credit, and we had to remortgage the house to clear debt."

She says she became more like an irresponsible big sister than a mother. Often she'd be out dancing till 6am while her oldest daughter babysat. "For two years I was a very selfish person."

The high was soon followed by a low. Slinger's marriage was failing, she moved into the spare room and suffered depression that resulted in two suicide attempts.

"He asked me to put the weight back on in a desperate bid to save the marriage, but I wouldn't have done that. And if you're going to ask me would I have lost the weight had I known how it would affect my family life, no I wouldn't."

She walked out on the family, and her husband refused to let her see the children for almost a year. They divorced, and soon after she met Mick. "A friend said, 'I'll introduce you to a friend – he's level-headed and you're off your head. I think he's the calming influence you need.' "

"Yeah, you were very neurotic when I met you," Mick says quietly.

Even though she kept the weight off, her life was still dominated by her body. It was after a tummy tuck operation went wrong and she almost died that she re-established her relationship with her children and, over time, found a new calm. Slinger knows her story is extreme, but believes few slimming groups warn people about the life changes that dramatic weight loss can lead to. "They don't prepare you for the fact that it might change your marriage, your relationship with your children, that you might emerge as a different person. And how are you going to maintain it, how are you going to be this person, and what will your relationship with food be like? When you go on these slimming programmes it's all, 'You're going to be slim, you're going to be wonderful'. They never identify the pitfalls, the baggy skin." She points to her upper arms and says she was lucky on that front – no bingo wings. All in all, she has spent £15,000 on fatty fallout surgery. There was the lower-body lift, the leg lift ("Just like pulling your stockings up") and the boob lift ("because they were down to my bloody knees").

Today she lives a quiet life, and is still on drugs for depression. She doesn't have a bank account, let alone a credit card – and for the first time in her life she feels content. "I've got a wonderful partner and I feel happy in my body at last. Even with the old scars, even naked, I'm totally comfortable in my skin."