With the benefit of hindsight Helen Beaumont is well aware that the signs of her husband's illness were clear long before his condition was diagnosed. “I don't know why I didn't see it,” she says. “But I was trying to hold on to a job, I had two toddlers, and Clive - he was so useless and I was so cross with him.”

A typical event was the time Clive, well-intentioned as ever, put the cashmere twinset knitted by Helen's mother into a hot wash. When, predictably you might think, Helen exploded with anger, Clive couldn't understand why she was upset. “That was what drove me mad - he always said nothing mattered. When our son went missing and we got him back he said it doesn't matter because there's no damage done, he's perfectly all right. I remember driving along with tears running down my face because it made me think how fragile things were.”

Not that the diagnosis would make things easy. When it came, three years after Clive started having difficulty reading and writing, Helen felt very alone. Clive had younger onset dementia, she was told in a brief phone call from a neurologist. She was offered no guidance or advice. Clive, her Oxford-educated husband who had recently taken redundancy as an major in the Army, was just 46.

What difference would an early diagnosis have made? “I wouldn't have spent three years fighting him,” Helen replies. “The children were 4 and 5 when dementia was diagnosed and old enough to realise the tension in the household. If I hadn't had small children I would have left him. He was intolerable. You can do so much more once you've got a diagnosis; we spent a lot of our time in irritation, misunderstanding and misery.”

Helen is not by nature a proselytising woman, but she knows that the frustration she experienced as she watched Clive deteriorate is far from unique, and that it was exacerbated by the association of dementia with people several decades older than Clive. In the UK around 15,000 people under the age of 65 are believed to have younger onset dementia, yet there is little provision for them. Clive died in 1999, six years after the official diagnosis, (like many people with dementia, of an infection), and it is to improve the lot of others with dementia that Helen has written her story.

Her account, Losing Clive to Younger Onset Dementia, is moving and practical, and very much a campaigning book; it is in no way a misery memoir. If anything, she underplays the grief that can still poleaxe her. A forthright Yorkshirewoman by birth, she talks of Clive without melodrama and paints herself as brittle and bad-tempered.

It is only when you see her at home in Abingdon, Oxfordshire, sitting on a sofa in a nest of knitting and books, that her vulnerability is tangible, though she fiercely brushes away any sympathy. “When people say, ‘Poor you, you've had a horrible time, I want to give you a hug,' I think, yes, that's nice, but Clive had a far worse time. That was the reason for telling his story. The last years of his life could have been so much better.”

She met Clive at a judo class when they were both at Oxford. “It was initially a physical attraction. He was just such a sexy, handsome, kind, considerate guy.” He joined the Parachute Regiment, they married, and while he was in his element throwing himself out of aeroplanes and later as an imperturbable bomb-disposal expert, Helen wrapped her career around her husband's postings and lapped up the scuba diving holidays they shared. When she was 36, their daughter Rachel, now 20, was born, followed by a son, Alan, the following year. They were happy and when Clive started to forget that he was due to babysit, Helen assumed that he was overwhelmed by the demands of his busy job and being a father.

After his redundancy from the Army - which would eventually, after a dogged campaign by Helen, acknowledge that he had been ill during his final years and agree to pay his pension - he spent a year applying for jobs and being rejected. Then came the diagnosis and, in the absence of any formal system of support, Helen could only improvise. Recognising that Clive was by nature an energetic man and a risk-taker, she allowed him as much freedom as she felt was safe for him and for others; she stopped him driving but allowed him to continue to cycle and swim. “Clive was the kind of man who'd enjoyed mountain marathons. I couldn't wrap him up in cotton wool. You can't change someone's nature. I did my darndest to keep him safe but I couldn't stop him going for a walk.”

The most powerful and poignant section of Helen's book is her description of Clive following her around their home as she prepared the family's evening meal. It is also a telling account of his limitations, the strain on her and the confusing emotions with which she was confronted.

“I felt so dreadfully sorry for him,” she says. “There was my wonderful strong independent husband and he's following me like a two-year-old, but at the same time he drove me mad. Every time you want to go somewhere he's there and you have to try to get round him. He could have lost his temper and beaten me up. He didn't. He hit me just once, walked out and came back an hour later as if nothing had happened. I felt so guilty that I'd provoked it, so relieved that he wasn't 18 inches away from me any more, and so proud of him. He knew what was happening and he broke the tension and walked away.

“I got through those years because I was strong enough to keep going, but it wasn't through choice. You learn to be calm, though it's just a front. I didn't want my children to go into care and nobody could think of anything to do with Clive so I did my best to carry on.” After four years of caring for Clive at home, Helen had no choice but to find a residential home for him. She succeeded, which meant that at least she could sleep, but then found herself caught up in a system designed to care for elderly people. She describes one occasion when they disappeared to Clive's room to make love - and found themselves tactfully ignored when they emerged later, to her great embarrassment.

“Sex was a very important part of our relationship and it was not supposed to be,” she says. “Clive's got dementia - how can he possibly still want to have sex with his wife? Well, why shouldn't he? When you build care homes you ought to build in space for couples to be together and to have some privacy so you don't have a situation where everyone knows why they're going off. It's a normal part of life.”

Her principal campaigning aim is to ensure that every primary healthcare trust has a recognised service for people with dementia and a consultant who is an authority on younger onset dementia; at present the care available depends on your postcode. She wants early diagnosis, and for patients to receive the kind of immediate and continuing support that is now given to cancer patients. She wants care to be appropriate for the age of the patient, rather than for someone 30 years older, and she wants younger onset patients to be counted. Helen is doing a master's degree that will enable her to engage in research into dementia. “I still meet people who have been ill for years before their dementia is diagnosed and they are told that nothing can be done, the door's over there. Once we start to get early diagnosis, we can count people and the better services will follow.”

She prefers not to dwell on her own loss and insists that other people have had it far worse which is clearly her coping strategy. That and The Clive Project, the charity she co-founded to provide friendship and care for those with younger onset dementia in Oxfordshire. It has already been copied and she would like to see its simple but focused practices extended throughout the country: from the point of diagnosis it provides a trained support worker who joins the individual each week in a pursuit he or she enjoys but could not otherwise do, whether swimming, playing golf or going to the pub. The spin-off is four hours of respite for the full-time carer.

“I haven't got over what happened to Clive,” she says. “I got an e-mail yesterday and would have stayed in floods of tears drinking wine if I hadn't dragged myself out for a walk. I can't forget and the only thing I can do is try to make something positive out of it. “It doesn't have to be as bad as it was for Clive and me.”

Losing Clive to Younger Onset Dementia: One Family's Story by Helen Beaumont is published by Jessica Kingsley Publishers at £12.99. It is available from Times BooksFirst for £11.69, free p&p, delivery within 28 days; 0870 1608080; timesonline.co.uk/booksfirst; www.thecliveproject.org.uk

Dementia: the facts

Dementia is the term used to describe a group of diseases that damage the brain, leading to a progressive loss of brain tissue. The tissue cannot be replaced so symptoms worsen over time.

Younger onset dementia is uncommon in under-40s, though it has affected teenagers. It is defined as affecting those under 65. Symptoms may include: loss of memory, difficulty concentrating, difficulty finding the right words or understanding what people are saying, a poor sense of time and place, difficulty solving minor problems.

It is estimated that there are more than 15,000 people under 65 with dementia in the UK, although the lack of research means that this could be a significant underestimate.

The author Terry Pratchett had a rare variant called Posterior Cortical Atrophy diagnosed when he was 59.

Most patients do not inherit the condition.

Younger people face particular challenges because they are likely to be working, to have dependent children and a mortgage, and to be physically fit and active.

At present, there is no cure. Drug treatment can temporarily slow or delay progression of the disease.

Alzheimer's Society, 0845 3000336; www.alzheimers.org.uk/ypwd

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