One night, shortly after Carla White had a blackout at work, she sat bolt upright in bed. "She woke up and said to me, 'I'm losing my brain,'" says David, her husband. "I think Carla knew straightaway. I almost find it eerie."

As their two children grew up, David and Carla filled their big house by the sea with visitors from Italy, where Carla was born. David played in a jazz band; Carla cooked and made clothes. David had dreamed of a retirement travelling the world together. Instead, Carla was diagnosed with Alzheimer's disease and David has devoted his days to looking after his wife. "Dealing with dementia is rather like a rugby tackle," he says. "You both hit the ground."

In the UK, there are 700,000 people with dementia, an umbrella term for diseases and conditions that affect the brain. Alzheimer's is the most common. Dementia is estimated to cost the economy £17bn each year. The number of people with dementia is forecast to soar to 1.7 million by the middle of this century but it already affects many more. The government this year launched a national dementia strategy, with £150m to train GPs to recognise early symptoms, establish what are called "memory clinics" to help sufferers and carers, and appoint dementia advisers to support families. Dementia charities, however, described the strategy as "a huge let-down" because of its limited scope. As campaigners point out, dementia remains severely underfunded and whether those with dementia and their carers get help is still a postcode lottery.

David was fortunate to live in east Kent where the services provided by the local NHS trusts, the county council and local charities have already anticipated the government's national strategy. But all the help in the world does not always ease the emotional strain of dementia - and our reactions to it.

Carla, an attractive woman in her early 60s who still speaks with an Italian accent, sits in a pool of sunshine on her sofa, looking out over the garden of their small bungalow in Birchington, Kent. She often seems lost in thought but when she smiles she lights up the room. David hates talking about her in front of her, as if she was a small child, so he retreats to the study to explain how their lives have been transformed.

He refers to Carla as being in her "eighth year" and measures her deepening dementia by the shrinking amount of time he can leave her alone. "It has come down from a day, to a morning to an hour and now a few minutes." For the first time, in the last few months, he has begun to struggle to understand her. "What can you do if you can't understand what your wife is talking about? My God, it's appalling," he says, quietly. "She's a very talented person, that's one of the problems now. She is an intelligent person so when things are going badly it makes it more painful, because she sort of knows in a way."

On good days, they do jigsaws together - "although she will take pieces out again" - and watch television. Carla is fascinated by classical history, particularly documentaries about the Romans. "She will say, 'I'm a Roman.' She will talk the whole way through it and I can't hear a word of the programme but I'm so delighted she is engaged with it." If he mentions coffee or red wine, Carla smiles: she still loves her coffee. When they go to church, she can sing perfectly in tune "but she can't do the words", says David. "I understand it's one of the last things to go ... the music."

On a bad day, Carla will ask the same question over and over again, or erupt in a rage. "I reckon I've got the carer thing sorted but I can't deal with the anger. She gives me the stare: it's horrible, hostile. The other night she woke up in the night screaming." He told the neighbours about her condition and hopes they understand. "The thing to realise when you have a bad patch is that this isn't Carla, this is the illness."

They fell in love after meeting in a City bar in 1979. David had just returned to Britain after a spell teaching geography in New Zealand; Carla was working in London. They married, had two children and returned to live near where David had grown up in Margate. He became head of geography at a local school, where he worked until he retired in 2003.

"The 1990s for us were really fun times," says David, a little wistfully. They travelled the world as a family and followed their son's and daughter's passions: music, sport and rugby. Unusually, they can pinpoint the moment when one life ended and another, very different, existence began: Carla had several blackouts at the factory where she worked in 2001. Doctors thought it might be epilepsy so she was put on a low dose of tablets. She had MRI scans and doctors sought to treat her problems with movement and balance. Meanwhile, she became more disoriented. She would take their cups of coffee to the kitchen but end up in the bathroom.

It took almost six years, until 2007, for Carla to be formally diagnosed with Alzheimer's. "With dementia, doctors are slow to commit themselves to say, 'Yes, this is definite.' So there was a bit of a delay," says David mildly. He is not being critical, although it was sometimes a struggle to get help. When he applied for a disability living allowance, the questions on the form related to physical disability and his claim was rejected. He had to go to court to get it awarded. He could not call on much family either: although their son is a regular visitor, their daughter lives overseas, while Carla's family is in Italy.

The formal diagnosis by a specialist in London hit David "quite hard", he says. His blood pressure soared. "You realise there is no way out, although you sometimes think there might be by walking out the front door." Gradually, simple things became more complicated. Carla needs a wheelchair when they go out. In the bungalow, he holds her hand when they walk from room to room. This year, he began taking her to the toilet because she has forgotten what toilet paper is. David does not want to consider what for many is the final stage: residential care. He won't mention the word. "We're nowhere near that situation at the moment where we would have to think about other things."

He always saw himself as a traditional "bloke with a beard" but he has learned many new skills, feels very well supported and is determined to focus on the good things they share. "There is still a lot of tenderness there. You do have these moments where she surprises you," he says. "If you get away from the frustration and upset and complete change in lifestyle, in a sense, what could be a better use of my time than looking after someone who is unwell?" He says it softly. "What a positive thing to be doing."

Veronique Provost-Appleby

The weekly carer

When Veronique Provost-Appleby was 16, she looked after her granddad, who was dying of cancer. Then she cared for her mother-in-law when she had cancer. Later, she helped her dad when he had cancer, and when her second husband contracted cancer, she looked after him too. "Story of my life, being a carer," she smiles.

David is ushered out of the living room, where Provost-Appleby is helping Carla with a painting. "It's not your time, it's girly time," she tells him. When she first arrived at their bungalow a year ago, "I just saw a man totally lost because for the first time in his life he had to deal with the day-to-day running of the house - washing and shopping." Now, she says, "he's not doing badly at all".

Provost-Appleby is with David and Carla for one morning a week. A care worker for the charity Crossroads, she supports six families. She helps David by caring for Carla, with whom she clearly has a great bond. "I can do things with her, things David hasn't a clue about - I do a manicure, I brush her hair, or put some lipstick on. When I get the perfume she goes, 'Ooooh'. Music is very important and touch is very important - the sensation of things. I've done her hands this morning and she loves the creams. She says, 'Oh, soft.' I said, 'Like a fish,' and she laughed."

Roger Newman

The family friend

David and Carla used to live down the road from Roger Newman and they would see each other at church. He was known for his work with the Alzheimer's support groups in the years after his partner was diagnosed with the disease in 1992 so, when Carla became ill, David sought his advice and they became friends. David says most of his closest friends these days are people in a similar situation: other local carers and former carers who make up a vibrant voluntary support network. In David's position, says Newman, old friends often fall away. "You discover who your true friends are. It is surprising who runs in the opposite direction.

"We live in a rational world and Alzheimer's is not part of that rational world," says Newman. He understands the guilt and anger that carers can feel. "Embarrassment is another one. You don't know what they are going to do. Sex - how am I going to deal with my sexual needs? There can also be financial issues which can be embarrassing and difficult." Perhaps hardest of all is "the realisation that when you turn the lights out at night, this is not going away. "People soldier on without getting any help whatsoever. Because people love each other they think that this is what they have to do, this is what love means, 'for better, for worse, in sickness and in health'."

David and Carla White are helped by a grand coalition of carers. But there will come a time when they both need even more support. "With dementia, there are various points when you fool yourself into thinking: we've sussed it," says Newman. "And then another stage unexpectedly rears its ugly head. That's what makes dementia such a bastard - you don't know what's going to be around the corner."

Jackie Tuppen

The Admiral Nurse

What struck Jackie Tuppen when she met David and Carla a year ago was David's loyalty. "He didn't want to do anything to upset Carla. It was like trying to draw teeth, getting him to say what the problems were. He doesn't like talking about anything that others may see as derogatory for Carla," she says.

She became a nurse 15 years ago and has always worked with older people. Later she specialised in dementia and, last year, became an Admiral nurse for Thanet, east Kent, the region where David lives.

Named after Joseph "Admiral Joe" Levy, a keen sailor who had dementia, there are just 60 Admiral Nurses in the country. They work not to support the patient but the carer, helping them to get the emotional and practical assistance they need. Although Admiral Nurses' wages are usually paid by partnerships of local NHS trusts, they would not exist without the support of the charity For Dementia, which funds their training, supervision and support.

Tuppen has no set time for visiting her clients: she is available whenever they need her. It was Tuppen who suggested David could use some help with bathing and now David is visited twice a week by another carer who bathes Carla. And she has provided David with emotional backing too. "There are different times when she has enabled me to manage my feelings of frustration," says David. He worried in the past that Carla is ill "because of me being a bad husband". Talking with Tuppen has helped put him right on that.

Tuppen explains how many carers wrestle with guilt for feeling frustrated. "In a normal marriage you would feel cross and grumpy with someone," she says. "People look at someone like David and think he's fine. Most carers get on with it. They don't talk about the dreadful time they are having. They keep it to themselves. I don't know if they feel there is stigma. There is still a lack of awareness in the public."

Sharon Sanders and Ross Nicholson

The day carers

Twice a week, David drives Carla to Westbrook House, an "integrated care centre" offering NHS facilities to help people recovering from operations on their hips or hearts as well as 30 day and residential spaces for people with dementia.

"As soon as I go in, it's, 'Hello, Carla,' and I feel like I'm some sort of alien from Mars," says David. He is very happy about this. "They don't even look at me. My ego is a bit deflated but they are doing their jobs properly."

Carla began coming to Westbrook House for two days a week a year ago. She also goes to Westbrook for respite - staying overnight for a week - which gives David a holiday. When she arrived, Nicholson talked to her with his rudimentary Italian. Now, whenever Carla spots him, she beckons him over. "She was teaching me the other day," he says. "I was holding up colours and saying, 'What's that?' and she would tell me and if I then got it wrong, she would correct me."

Each client is treated as an individual and their dementia takes vastly different forms. It is a stereotype, says Nicholson, to imagine they all regress into childhood. "They may appear to be childlike but you shouldn't assume that they are; you don't know what's going on in their mind."

Although not everyone with dementia has outbursts of anger, Carla does get upset. "Usually talking to her quietly and listening to her calms her down," says Sanders. "One day a little while ago she was quite upset several times. I wondered if she was having an off day, like we all do."

• Postscript: Roger Newman's comment that carers can be confounded by the rapidly changing stages of dementia proved prophetic. Some weeks after I interviewed David and Carla, David took a two-week holiday to visit his daughter and, when he returned, learned of several incidents at the care home where Carla had spent her holiday. "She had got to this aggressive, angry phase and she started waking up in the night, screaming," he says. He took her home, and for a whole day, she refused to eat or go to the toilet or do anything, except laugh and scream at her husband. In despair, David called social services. Carla was found a place in a privately run care home nearby. "I said to her several times, 'I'm sorry, but I can't do it any more.'"

David is adapting to life in an empty home. He admits he is "quite pleased" that friends are calling and his interests are slowly returning. Every few days he visits Carla. "One time she pulled my hair and told me to go and another time she gave me a cuddle," he says. "She will sit there and chatter away but she is not saying anything you can respond to. The first time I visited she said something about coming home but now she never mentions it. I can sit there for half an hour and hold her hand. When I leave, there is no scene. I say, 'See you next time.' There's no point in saying 'tomorrow' because she no longer understands what it means."

• In Sunday's Observer: How the care system is failing people in hospital, care homes and their own homes. And find the full Ageing Britain series at theguardian.com/ageingbritain