He was sweet and loving to the outside world, but John Thaw could make life a misery for his family. Sheila Hancock tells Arminta Wallace how she and her husband overcame his alcoholism

Celebrity. We so take it for granted that it's easy to forget how strange it is. When two actors got married in a register office in London in 1974 the female half of the couple was famous: tabloid-front-page famous. The evening papers gleefully reported the ceremony, complete with huge photographs of her - and tiny ones of him. Shortly afterwards she - a celebrated stage actress who had made some daring forays into alternative comedy - received an OBE. He was still an unknown quantity, so much so that the entry under John Thaw's name in the Who's Who for the following year read simply: "See Sheila Hancock".

All that, Hancock notes wryly in her new memoir of the marriage, The Two Of Us, was soon to change. The book is a chronicle of change. It charts the movement of two young working-class people into a world of glittering prizes. It charts Thaw's rise to television superstardom, first as rough, tough Regan in the cult detective series The Sweeney, then as the irascible anti-hero of Inspector Morse. But change, especially when celebrity comes into the equation, isn't always good. For this celebrity couple it proved a treacherous business. Hancock goes on to record her husband's descent into alcoholism and their 18-month separation. Then he got sober and they had a blissful reconciliation until he died of cancer, in 2002, at the age of 60.

In Belfast to promote the book, Hancock sits calmly on the other side of an enormous boardroom table in a city hotel while I grope for a way to ask the first of a long list of personal - sometimes horrendously personal - questions. She is tall, cool, elegant. Why, I want to know. Why tell all, in such a public way? "I was thrown into it," she says. "It was a sort of self-defence mechanism, writing this book. I didn't want anybody else to write it. Other people were threatening to write about John, and I thought, OK, there are things out there that I'd prefer to talk about rather than anybody else." But what would he . . . ? She raises her hands in an eloquent shrug. "I don't know what he would think," she says. "I have no idea. He might say 'Brilliant, kid' or he might say 'How dare you?' I've no idea. I don't like that stuff: 'Would he be upset?' Or here's another one: 'He wouldn't have wanted you to be unhappy.' Would he buggery not have wanted me to be unhappy. He'd have been absolutely livid if I was skipping around happily after he died." Ever the actress, she pauses for just a beat while we chuckle at this. Then she continues, quietly: "But he's not here. So I must do what I feel is right. And I would be lying in my teeth if I wrote this book and said our marriage was sweetness and light all the time."

Searing honesty is not the only thing that sets The Two Of Us apart. There is also its structure: it is a cleverly balanced tale of life and death, with raw, jagged diary entries from the time of Thaw's final illness interwoven with chapters devoted to the eventful, character-strewn life stories of both partners.

She looked up newspapers in libraries, tracked down acquaintances of Thaw's whom she had never met and cornered mutual friends with a tape recorder. "Actually, getting them to be honest about what he was really like at work and so on, and tell me stories about him, I got really intrigued. It was almost as if I was writing about somebody that I didn't know."

Hancock's ultimate act of research was to find out what happened to Thaw's mother, Dorothy, who had been out of his life since she left when he and his brother were children. Hancock's portrayal of a dreamy young woman who couldn't hack the drudgery of marriage and children is sympathetic, begging the question of what Thaw would have thought of it. She shrugs again. "I felt I owed it to her. I thought, I'm looking at all the other areas of John's life, I can't just condemn this woman as an evil cow. I'm not a great believer in respecting the dead if what they did was not necessarily the right thing. And John's hatred of his mother" - she stops as if about to correct herself, then nods grimly - "yes, it was hatred sometimes . . . was not a healthy thing. And he wouldn't even look at it. I often tried to get him to talk about it, just for his own sake really. So I thought, well, I may end up hating her as well, but I've got to try and understand how she could leave her children."

The core of the book, devoted to what Hancock refers to as the drinking days, will shock anybody who followed the adventures of Morse with uncritical admiration. Morse, you will remember, was fond of a pint. Or even a couple of pints. In Thaw's case it was a lot more, but very few people knew that. Even his colleagues on the programme, it seems, didn't realise the extent of his alcoholism. Isn't it a kind of family betrayal to bring it into the public arena now he's dead? Hancock smiles sadly. In the first place, a journalist was threatening to write an unauthorised biography of Thaw, which might have said almost anything. And, in the second place, when her eldest daughter read the first draft of the manuscript she insisted her mother be more forthcoming about how awful their home life was. "I would have been much more reticent," she says, "but her theory was that if I didn't show it almost as bad as it really was then his recovery wouldn't mean anything. I mean, the miracle of what happened when he came into sobriety. It was a miracle for all of us."

Is it possible that her children were angrier than she was about the alcoholism? "Yes, I think they were angrier, because they're more grown up than me," she says. "They thought we were ridiculous. Absurd. And they did go through times when it was difficult for them. Not only John's drinking but my preoccupation with it. You can be very neglectful of your children when you're obsessed with somebody's irrational behaviour." Irrational is putting it mildly. At a birthday party of one of his daughters Thaw and Dennis Waterman, his friend from The Sweeney, got very drunk in the basement. Every time Hancock went down for supplies he harangued her. "It was like an Ayckbourn farce," writes Hancock. "All gracious smiles for the guests upstairs, and spitting venom in the basement."

There was worse to come. When Hancock was diagnosed with breast cancer, in the late 1980s, Thaw refused to discuss it or accompany her to a doctor. Her daughter found her a house to rent, and she moved out. Newspapers asked how Hancock could desert her sweet, loving husband; one pictured a gloomy Thaw in a superimposed apron, ready to minister to his sick wife. How far this was from the truth is starkly illustrated by a note she wrote to him: "You are deeply loved and we long for you to discover how good life can be . . . . It is terrible for me and the girls to watch you being so, so sad. You feel lonely and persecuted but you only have to ring one of your daughters and they would be there. They daren't ring you, sadly."

After all that, the sobriety - achieved after just a week of consultations with an Irish Harley Street guru by the unlikely name of Beauchamp Colclough - does seem a miracle. "John described his sobriety as like a cloud lifting," writes Hancock. He never touched another drink. When the couple got back together they turned, she declares, into a pair of old farts. The mutual affection that suffuses these pages is palpable. Picnics on beaches and in parks, little notes and little presents, shared jokes, listening to music together: it is a portrait of magic shimmering through the apparently mundane.

"Oh, we were good at the old picnics," she says, grinning broadly. "Who could make the best coffee. The search for the perfect cup of tea. You know, finding new brands in Fortnum & Mason. He did love listening to music, just the two of us," she says. "It was his idea of bliss to sit with a tray and watch EastEnders. Pathetic really." Is she kidding? "Well," she says, "what was very poignant, when I was researching the book, was looking back at times when he said he loved me and things like that. When you look back at those you're apt to think, well, why didn't I enjoy it more at the time? But of course at the time you were worried or embarrassed, or whatever else, and not concentrating on the moment. A couple of weeks ago, at one of these funny book signings you have to do, a woman handed me a piece of folded paper. I opened it and it read: 'Cherish the moment.' That's one of the things I'm trying to say in this book. Cherish the moment."

Another aim, she says, was to write without sentimentality about bereavement and grief. Still another was to boost funds for the John Thaw Foundation, which funds small projects, mostly to do with underprivileged children and those with learning disabilities.

In her younger days Hancock was a staunch political activist who could be relied on to turn up at protest marches. She is now a committed Quaker. "I do quite a bit of prison visiting and some Quaker work - not as much as lots of other people, though." Is she still, at 70, an angry young woman? "I'm very angry at the moment about what's happening. I'm angry that nothing changes, really. I'm not as active as I used to be - my children have suddenly become quite militant in various ways, which pleases me - and I infiltrate my grandchildren's brains every now and again. But I'm not marching the way I used to." There is an uncharacteristic pause. "And I should be, because there are plenty of things to march about. I'm very thrilled about Europe and unification, and the threats of my childhood, such as war with Germany, are absolutely gone, and that's brilliant. But there's always some new ghastly thing, and I'm afraid I've got to the age when I know that it will always be like that. That's what we are. We're human." Humanity. For all its warts and hiccups, it makes celebrity look pretty tawdry.

The Two of Us: My Life With John Thaw, by Sheila Hancock, is published by Bloomsbury, £17.99