Jane Hawking was devoted to her husband Stephen - until he left her for his nurse. In a rare interview, she remembers the idealism of their early years together and how their marriage went so wrong

A few weeks ago a little package of time past was posted through the door of Jane Hawking's neat detached house on the edge of Cambridge. It contained a video of the BBC's forthcoming dramatisation of the courtship and marriage of Jane and her first husband Stephen Hawking in 1963, when sexual intercourse had just been invented. She had been involved in the early stages of making the film, which is based in part on her book about their marriage, but still so far she has only managed to 'watch a few bits of it'. She can't, she says, quite bring herself to sit through the whole thing, 'given what's gone on'.

That 'given what's gone on' could take in any number of things from the 40 years since the events the film describes. It could refer to ways in which that early romance eventually hardened into something complicated and angry. It could include a reference to the acrimonious separation and divorce that followed, when Stephen left to live with his nurse, now his second wife, Elaine Mason, and Jane married her choirmaster and long-time family friend Jonathan Hellyer Jones. But specifically it seems an allusion to the recent months of headlines and anxiety that have focused on the police investigation into alleged mistreatment of Hawking, who was left out in the sun in his wheelchair on the hottest day of last summer, suffering severe sunstroke to add to the cuts and bruises reported by nursing staff. In January Jane Hawking was saying: 'The police can't be allowed to drop the investigation.' Last week, though, the police dropped the case, moved apparently by Stephen Hawking's refusal to make any complaint. The nurses who made the allegations are no longer employed by the scientist and his wife. And Jane Hawking, who seems slightly in a state of shock about the turn of events, is quietly adamant, 'for legal reasons', that she does not want to go over that ground at all in this interview, so we sit in her conservatory and talk about that bit of time past.

'Last September I went over to watch some of the BBC filming taking place in Trinity Hall,' she says. 'And they were doing a good job. I was very impressed, overwhelmed in fact at the sight of all these huge vans and hundreds of people blocking up the streets of Cambridge, for what exactly? For this drama about these two years in the lives of Stephen and me.'

Jane Hawking is precise and bright, but as she talks about these things she can't help but give the impression that her life with Hawking has left her a legacy of deep regret and, though she tries to hide it, of bitterness. The film is dominated by the brilliant physical impression Benedict Cummerbatch gives of Stephen. 'It was uncanny,' Jane says. 'He'd worked so hard in researching the exact progression of motor neurone disease. It brought back that period so very strongly.' She smiles a little. 'I think the young lady playing me, however, was much more feisty than ever I was. I was always extremely determined but I was also quite timid. So in that sense many of the things that appear in the film are not quite historically accurate.'

What the drama does capture is the extraordinary optimism that she and her young husband found with each other. He was just 21 when they met, trying to find a subject to direct his brilliant mathematical intelligence towards. Almost immediately after they got together he discovered that he was gravely ill, and not long after that, that he had only a couple of years at best to live. 'That is what should be most important about the film,' she says. 'That sense we had that, despite it all, everything was going to be possible. That Stephen was going to do his physics, and we were going to raise a wonderful family and have a nice house and live happily every after.'

Certainly, as the film shows, she went into their marriage knowing the worst was almost certainly around the corner. 'Yes, but at that stage I did not want to think about that. Also, we had this very strong sense at the time that our generation lived anyway under this most awful nuclear cloud - that with a four-minute warning the world itself could likely end. That made us feel above all that we had to do our bit, that we had to follow an idealistic course in life. That may seem naive now, but that was exactly the spirit in which Stephen and I set out in the Sixties - to make the most of whatever gifts were given us.'

Jane took much of her dramatic hope at the time from her faith, and still sees something of the irony in the fact that her Christianity gave her the strength to support her husband, the most profound atheist. 'Stephen, I hope, had belief in me that I could make everything possible for him, but he did not share my religious - or spiritual - faith.'

The story of Stephen Hawking's survival in those years, and his triumphant inquiry into the origins of the universe, was, in many ways, as she recalls it, the result of a spectacular fission of their opposed philosophies. 'Stephen's belief was that if you were free to do your absolute best work you would be rewarded. My belief was that if you gave all of yourself, to what you believed was right, then that would be enough.'

I wonder, given all that has gone on, and all that continues to go on, if it is possible for her to separate how he was then from how he is now.

'Yes I can. I can remember vividly the euphoric sense we had about us, that we were doing something exceptional. Once I had written my book, Music to Move the Stars, and I had exorcised some of the worst times; now I feel I can get back to that.'

The worst times, which her book detailed, revealed the way in which her husband, in her terms, became remote and impossible, an 'all-powerful emperor' and a 'masterly puppeteer', making a good deal of their life together a misery. Given Hawking's almost saintly public profile it is odd to have to face this portrait of him. Most difficult perhaps is the assertion that he could not begin to come to terms with his illness.

'All along I suppose I tried to imagine his feelings,' Jane says, 'because he would never ever talk about how he felt - he would never mention his illness. It was as if it did not exist.'

This refusal, she suggests, also prevented him from taking any palliative steps that might have made their early marriage a little easier. 'I had two tiny babies, I was running the home and looking after Stephen full time: dressing, bathing, and he refused to have any help with that other than from me. I thought that to coerce him into taking these measures would have been too cruel. One of the great battles was getting Stephen to use a wheelchair. I'd be going out with Stephen on one arm, carrying the baby in the other, and the toddler running alongside. Well it was hopeless because the toddler would run off and I would be unable to chase. So that kind of thing made life rather impossible.'

That refusal to countenance the disease, does she think it has helped him survive it?

She says she is not sure about that and mentions instead recent research which suggests that 'it seems motor neurone disease does far more damage to the parts of the brain that concern emotional reactions and conscience and personality than was ever thought, in some patients anyway'.

Things, for her, went from bad to worse after the publication of A Brief History of Time. 'Fame and fortune muddied the waters,' she says, 'and really took him way out of the orbit of our family.' Just before their marriage broke down after 26 years she revealed to a journalist that her role with her husband no longer consisted of promoting his success but of 'telling him that he was not God'.

Does she still feel it was like that?

'Certainly that he felt he was omnipotent, you might say.'

In response, not surprisingly, Jane Hawking felt she had to create a life of her own. 'Living here in Cambridge you had to have an identity. It was not enough to be a wife. So I did a PhD in medieval Spanish poetry. Stephen did not have too much time for that. I guess when you are thinking about the origins of the universe these things do not matter much. But I still find it all absolutely enchanting. It hasn't led to a career, of course, although I have done some sixth form teaching, and some university teaching, and in a sense the frustration is greater now than it ever was because I feel I have had a great deal to offer but I have nowhere now to go.'

She continues to find, she says, several times, great love and support and happiness from her second husband and the three children she had with Stephen. 'Without Jonathan, I would have gone under,' she wrote in her book. 'I would have been at the bottom of the river or in a mental institution.'

Does she still see Stephen?

'I used to see him. I never set foot in his house, of course - that is very much forbidden territory. But I used to go and see him in his office, and we used to have a good time, talking about the children and then about William, our grandchild. But I don't even know now whether he is in hospital [where he has recently been treated for pneumonia] or back at home. The children don't know either. So that,' she says sadly, 'is where we are.'

Does she feel she will ever be able properly to start the new life she promised herself?

'Well of course I don't think any of the past will go away. The thing is, with me and Stephen for many years I put every spoon of food into his mouth, dressed him and bathed him. You do not forget that experience.'

Still, the idea of her writing her book five years ago was to get it all out and hopefully be able to get on with things. She laughs bleakly at the idea. 'I thought it was terribly important to document that life with Stephen. I did not want someone coming along in 50 or 100 years, inventing our lives. Already that has begun to happen a bit with this film, but at least I think it is true to the spirit of it.'

That spirit, of a young couple lying on their backs on the green grass of Cambridge, looking up at the stars, is no doubt hard to reconcile with the reality of the 40 years that have followed, and that is what Jane Hawking lives with, the extraordinary ways in which their dreams went right and wrong. 'We were,' she says, 'great ones for taking a chance on life, I suppose.'

· Hawking is on BBC2 on 12 April at 9pm