'I knew it was you," says Carole Caplin. "I knew it was you because you don't look like a member." I have just walked through the door of a gym in north London where Caplin, 44, is a fitness consultant. She is sleek and well groomed and her face is set like a jelly. I am inclined to approach her with caution. The former style adviser to Cherie Blair has a reputation, as impressive as it is mysterious, for getting otherwise competent women to submit to her authority. One minute they're defending a client at the bar, the next - is it flattery? Bullying? Voodoo? - they can't apply lipstick without her involvement. "You don't look like a member," she says and it sounds like code for "Oh my God, you're a total wreck and only I can save you."

It is two and a half years since Caplin's ex-boyfriend, a conman called Peter Foster, advised Cherie Blair on the purchase of two luxury flats in Bristol and plunged all three into scandal. It seems like ancient history now, but the question of Caplin's friendship with the prime minister's wife remains intriguing. What did the barrister see in the fitness fanatic? Who had power over whom? Do they still talk? Is Caplin's mother really psychic?

Much of the coverage at the time ridiculed both women and Caplin, at least, is a convenient rendezvous for all sorts of gripes about modern life, from its overemphasis on image to cronyism within politics to complementary medicine. As Caplin doesn't give many interviews, the mystery of her appeal has deepened. In Vanity Fair terms, is she an unthreatening, Amelia Sedley type, employed because hanging out with her can only enhance one's own self-esteem? Or is she Becky Sharp, the social climber, who understands with the cunning of an arch arriviste that bossy people like, occasionally, themselves to be bossed?

Right now, Caplin has a television show to promote. She says before the interview that she won't talk about the Blairs, but when the subject comes up she offers no resistance. Caplin takes her work extremely seriously; whatever else one says about her, she is ambitious and hard-working, frequently at the gym by 7am to hold a class. How does her show, the Carole Caplin Treatment, differ from the Trinny and Susannah template for makeover telly?

"I guess at LifeSmart [her company] we never just have one dimension. So if you're sitting down in front of me, it's not about me saying, well, hang on Emma, you need to do this, that and the other, I don't like the way you're dressed or, you know ..." there is a slight pause. She leans forward in her chair. I lean back. Caplin is wearing a Stella McCartney suit and is styled to within an inch of her life. I am wearing jeans and haven't washed my hair since Tuesday. "I mean, personally it's none of my business," she says, snapping out a smile. "But if you sit down and say to me, Carole, I've had an ongoing problem ... my energy levels are plummeting, my sleep is disturbed ... Then suddenly my interest is piqued and it's like, OK, we need to go detective on this. "

"Going detective" means finding out through a combination of dietary analysis and amateur psychology what a client's problem is, and making lots of sensible suggestions on how to improve their health and fitness. Caplin is against what she calls cover-up jobs and although she is a big advocate of complementary medicine - one of the reasons she drives a 4x4 is that she has to cart so much holistic stuff around - she is not a crank. She wouldn't, for example, try to help a cancer patient without liaising with his doctor first. "The thing about cancer, Emma," she says, "is that you have a risk of it spreading."

She found the experience of filming quite difficult - "I wasn't the kind of person that could necessarily be scripted" - but adjusted and is pleased with the result. Of course she understands that elements of the press will continue to ridicule her; for all she knows, she says darkly, I might be one of them. "When I said yes [to this interview], friends said, 'Oh, are you going to go and read her stuff?' and 'I've heard ...'" She leaves a pregnant pause. Heard what? Caplin assumes a saintly expression. "And I said, you know, I'm not interested."

Caplin says: "The fact of the matter is, you're going to write whatever you want to write, you've got a brief, I don't know what that brief is, I can't compete with that. But I can be myself."

All I can think to say is, "Yeah."

There is a short pause.

"I find it hard ..." I begin. "You do?" she says eagerly.

"No. I find it hard to understand your decision to write for the Mail on Sunday given what a hard time they gave you and your friend Cherie."

"Oh." Caplin has an agony column in the paper that dispenses good and often quite funny advice to Mail readers. To the man who wrote in asking if it would be OK for him to ask his girlfriend to take part in a threesome, Caplin replied sure, as long as he was prepared for the third party to be a man. Still, why write for a title that so comprehensively trashed her three years ago?

Because, says Caplin with divine logic, not writing for them would constitute "going the victim route". And anyway, the Mail on Sunday was never as bad as the Daily Mail. She is using the column to preach her position on the EU food supplements directive, among other things, "And I thought that that was far more valuable than me picking holes and being offended by what the other paper did."

Well, financially, yes. But Caplin says she had to make a living somehow. After the scandal, she couldn't take on any new clients for fear they were tabloid hacks in disguise. She turned down I'm a Celebrity. She turned down what she calls "the £1m book deal", an honourable move for which, by virtue of raising it, she would clearly like to be thanked.

"I think it's like anything, Emma, you either compete and you get into this war situation, or you understand what the game is about, and actually it's nothing to do with me." It's the smartest thing she's said so far. "If you've got a brain and the wherewithal, you can hold those things together."

She means collaborating with the enemy. I tell her this sounds like political thinking to me.

"It's survival."

The Cheriegate affair

The greatest influence in Caplin's life has been her mother, Sylvia. Descended from Russian Jewish immigrants, Sylvia's own mother died only recently at the age of 103. "Great women in my family," says Caplin. "Sylvia is a very strong woman. She hasn't had it easy."

Sylvia divorced Caplin's father, a furrier and gambler, when Caplin was still a toddler. It was a time, she says, when "Divorce didn't happen. She was an outcast. She said bollocks to this, and went to start teaching in a little dance studio. She taught tap. And Mia Farrow was her client, and Clive Dunn." The mind boggles. Were they in the same class?

But there's no interrupting. "She was the Daily Mail's darling. She did the Shape Up and Stretch with Felicity Kendal. She started our industry. She was the first woman to take it public. She got the Dance Centre off the ground, she got Pineapple with Debbie Moore off the ground. She helped Daley Thompson get through his injuries to win all the stuff he won.

"And she choreographed the film Julia, with Jane Fonda and Vanessa Redgrave, and taught Jane, outside of that."

It takes a certain level of chutzpah to name-drop as shamelessly as this. Caplin reminds me, rather endearingly, of Charles Ingram, the cheating major from Who Wants to be a Millionaire? Apart from Kendal, Moore, Thompson, Fonda, Redgrave, Farrow and Dunn, she takes in Quentin Crisp, a whole slew of "very good friends in the media", and at the top of the tree, Cherie, who is introduced thus: "My mother," says Caplin, "had a massive celebrity client list. Janet Street-Porter was one of them, which makes me laugh. Because Janet then ended up getting on the bandwagon and slagging her off in an article when all the Cherie-gate thing happened."

Cherie Blair met Caplin when the latter was still in her 20s and holding a wellbeing class which Cherie attended. After leaving school, Caplin had taught some of her mother's dance classes; then she went into events management, but her interest in health and fitness overtook her love of showbiz (eventually, she would find a way of combining them) and she wound up teaching fitness classes and advising people on how to make the most of themselves. In any case, she says, she and Cherie were friends long before Blair become prime minister, so there is no question of having cynically "targeted" her.

I am curious about their friendship. Although Caplin insists that, "When high-powered women come to me it's not because they feel inadequate," the way she refers to them is terribly infantilising. She talks about doing "remedial" work with clients; about how 95% of them are business, legal and media people: "They are very bright and intelligent, so you're learning stuff off them and they've got the intelligence that I'm giving them on top of their own. That more than makes them equal."

She talks about the "hand-over" of a person's image when she thinks they can start managing their own wardrobe again. "I like styling people, but only if they can take it on for themselves." She looks sad. "But some don't. They just don't get it right."

In Cherie's case, she says, the truth is very boring. She can count on the fingers of two hands how often she socialised with her. It was always business, which came in short, intense bursts and then they wouldn't see each other for months. "It wasn't glamorous. It was like looking for stuff that doesn't crease, because of the travel; looking for stuff that when she's very tired, allows her colour to come out more. Making the best of that body, knowing that [she's] going to be photographed, knowing that there is a brief to get her with her mouth open. Making it as simple and elegant and plain as possible, rather than flowery and flouncy and A-liney." It is also about "being able to get the weight down" - this sounds tremendously bossy. "There were some quite big changes ... You can only try to help them develop an eye. You try to neaten up and tidy up."

Of course, pesky rivals will get in one's way on a project of this size. "You get other people who come in and they put too much makeup on and you've got to go in and say" - Caplin takes a deep breath and exhales - "less is more."

Lancing the boil

Peter Foster, the man with a criminal conviction for, among other things, conspiring to supply a special weight-loss drink that turned out to be tea, was Caplin's boyfriend and the father of a child that she miscarried when the Bristol flat episode blew up. She can't believe she was taken in by him, although she says those friends of hers who met him thought he was "lovely", too.

"I still cringe when people say my 'ex'. It just brings back the cringe factor and makes me think, oh God, what an arsehole. Me, I mean."

Well, he was one too.

"I can't speak for him," she sniffs.

Caplin says she was "gutted" that Cherie was tainted by association with Foster but, "There was never any blame."

Contrary to the way it was reported, Caplin says it was she, not the Blairs or their advisers, who made the decision to stop her coming to No 10. "I made a call and I said, 'Look, this is nothing personal but I can't go back to Downing Street. Or Chequers. Because every time I come [to visit] there's a double page spread about this person who's got power over you guys! And there's only so much I can sustain.' And they absolutely understood."

Perhaps I look stunned, or amused. Either way, Caplin seems at the last minute to realise how this sounds, and corrects herself. If she hadn't "made that decision" she says, Downing Street would probably have "banned me anyway. Christ, I would have." And she smiles.

One good thing to have come out of the episode, apart from the column in the Mail on Sunday, is that it has "lanced the boil" as far as her dodgy taste in men goes. She still calls herself a friend of Cherie's, but they don't see each other, at least not in public; it's too difficult. "I've always been a very private person," says Caplin. "I mean, can you remember a picture of me in a restaurant with somebody?"

I point out there was a picture of her in the London Evening Standard diary the night before. "Oh. That was ... I was ... I love the theatre and that's my one respite. And I'm very good friends with Jenny Seagrove because we've run a campaign together. So often, I will go to an opening."

She would describe herself, she says, as quirky and, "Unconventional. I'm not married, I don't have kids. I'm not falling off the arm of a man. But I love my life." And, she says, people can take it or leave it.

We finish up and go out for the photographs. Caplin says she is camera shy, then offers to drape herself over a huge exercise ball. The photographer gulps and tries to adjust. She is in many ways ridiculous: a proponent of natural health who has had both Botox and a nose job ("I don't see a contradiction"); a martyr to bad press attention who loves namedropping her contacts in the press; someone who calls herself a "typical Capricorn". But her mother isn't a psychic; she doesn't know one end of a crystal from the other. And to my surprise, I can't help liking her.

· The first Carole Caplin Treatment will be shown on C4 on Monday January 16, at 3pm.