Lucy Hawking's fears

Writing the book provided an escape, not only from her difficulty in maintaining a relationship with her father but also from coping as a single parent to her six-year-old son, William, who was diagnosed with autism three years ago.

The previous year, Lucy had separated from her husband, Alex Mackenzie Smith, a civil servant working in Bosnia. "William does see his father regularly but it was hard coping with his condition on my own. The worst time was two years ago - he wasn't sleeping, was having regular tantrums and could be very unmanageable. I felt very isolated and withdrawn.

"Jaded was my escape into a fantasy world. I did think: 'Why me, why my little boy?' I felt like saying to God: 'How can you do this? This is so unfair.'

"When William was diagnosed, the doctors said there was not much they could do for him. I lost hope but my mother kept saying there had to be something we could do. In the end, she found a paediatrician who prescribed a course of treatment that has had a huge impact on him.

"Just having the determination to persevere, to find a way of doing something - which I think is what my mum did with my dad - is a big lesson she taught me about life."

Lucy says she was a happy child. She grew up in Cambridge with her brothers Robert and Tim. "I was never bullied because my dad was in a wheelchair," she says. "Either I didn't notice, or people were terribly nice, but it was never a factor in how people treated me.

I felt quite normal even though I knew my home life was different from my friends'. I would go to their houses and there was a calmness that there wasn't in our house."

Her mother, Jane, Hawking's first wife, deserves the credit for heroically keeping everything together.

"She worked very hard and every day she'd get up and do it again. It was an unstinting endeavour on her part, and she was doing most of it by herself. I see now what an enormous part of her life she gave up to look after my father."

Lucy says she never felt put upon to look after her father although she and her brothers "did more around the house than other children would. I would cut food up for my father and help him eat".

There was little physical contact between her and her father - "It's hard to say if I missed it," says Lucy, "because I never had it. I don't think I worried about it particularly, it wasn't a priority" - but the children, perhaps more sensitive than most children their age, made an effort to include their father as much as possible.

" Every Sunday afternoon we would play board games with him or go for a walk," she remembers. "I didn't feel resentful, I just wished that he could walk and I wanted him to be okay."

As Hawking's disease progressed, he needed round-the-clock nursing. "That was very difficult because our house became a hospital. There were times when I'd be reading the paper at breakfast and one of the nurses would come and whip it out of my hands and say it was for my father. They would just take over. It was an invasion."

Hawking's body may have been degenerating but his mind was as brilliant as ever. As Lucasian Professor of Applied Mathematics at Cambridge University, a post he has held since 1979, much was made of the brilliant mind, supposedly capable of "thinking in 11 dimensions" trapped in a body memorably described by his first wife as that of "a Holocaust victim with the needs of an infant".

The phenomenal success of his book A Brief History of Time - 25 million copies sold worldwide - made Hawking a celebrity. "We weren't a normal family but we had balance, even if it was a precarious balance. But then a combination of factors - the nurses, sudden fame, travelling a lot more - changed the dynamic in the household. Fame would change anybody and it did change him."

The Hawkings' marriage had been under strain for some time. Jane had started an affair with Jonathan Hellyer Jones (to whom she is now married), her choirmaster and Lucy's piano teacher, with her husband's knowledge and agreement. But it was Stephen, who had fallen in love with one of his nurses, Elaine Mason, who actually ended their marriage in 1989.

"The fame and money arrive and - bingo! - dad's gone," says Lucy. "It's very difficult to talk about because being bitter is so unpleasant and not helpful but I think it's undeniable that there was an element of that at the time."

It is clearly a difficult subject for Lucy. She curls into herself on the leather chair and suddenly all the chatter is gone. When I ask if she attended her father's wedding to Mason in 1995, she just says "no" and stares at me, willing me to move on.

Does she have any kind of relationship with her father's new wife? "I can't comment on that," is all she will say. "I would be really happy for my father if I thought he was happy."

The implication is that he is not happy but one of her problems is that Lucy can never talk to him in private. "I've worked very hard to maintain a good relationship with him. I see him as much as anyone my age sees their parents, every few months, sometimes longer if he's away.

"I certainly feel that access to him is restricted, that it's not possible to see him as much as I would like. I feel a bit sidelined. There's always a carer in attendance or other people around."

This would make talking frankly about the abuse allegations impossible, even if Hawking wanted to. Lucy and her family have lived with their fears for a long time but they were made public only four years ago when the authorities were contacted over claims of mysterious injuries.

An investigation was dropped after Hawking refused to make a complaint. Earlier this year, another police inquiry began after Hawking was reported to have been cut, beaten and left alone in his garden on the hottest day of last year, suffering severe heatstroke and sunburn. But he denied he had been mistreated. The inquiry was abandoned.

"It continues to be a subject of great concern," says Lucy. "Especially when someone is very vulnerable. But my father disputes the whole idea of him being vulnerable. I can't say if I would like the investigation reopened - he doesn't want the police to investigate his life, he sees that as an invasion of privacy - but I am keeping an eye on the situation."

Lucy succumbed to the pressures in January this year. She took her son to her mother's house and checked into The Meadows, a rehabilitation clinic in Arizona.

"I was in a state of full-on mental and physical decline. My father was in intensive care and everything got too much for me. I just couldn't see how I could get myself better.

Before, I'd had bad times but always felt I could pull myself out of them. This time I thought I actually needed someone to help me.

"Christmas was very stressful because my dad was ill. I'd been drinking quite a lot before then but I had given up. But then I thought it would be a good idea to have a glass of wine, and I fell to pieces.

"I felt very confused and distressed, as if all the connections in my brain had failed. So there I was in Arizona, where I promptly managed to break my hand while playing basketball. I thought: 'Oh my god, I'm such a reject, I even manage to break something in rehab.' "

She laughs when she says this but she does admit to battling bouts of depression, not least when dealing with her son's autism. "Sometimes in life you come across moments when you understand the true meaning of the clichés you hear all the time.

"The day I was told William had autism I actually felt as if my heart was breaking into tiny pieces. I will look after him for the rest of my life if I have to. But the great ray of light in my life is that, against the odds, William is getting better.

He skis, swims, rides horses, and so on." She laughs. "Something in this story has to have a happy ending." That might just be Lucy Hawking the novelist talking.

Jaded by Lucy Hawking, £6.99 (Review) is published on 26 April.