The writer Margaret Drabble lives in a Hampstead house and Michael Holroyd, the husband to whom she is devoted, lives in Ladbroke Grove. It is an arrangement that London’s top-drawer literary couple have maintained since their secret wedding in 1982. But now she wants to move in with him. When they entertain, they sleep at her house; when they go to the airport, they spend the night at his. ‘We speak every day,’ says Drabble of Holroyd, the enigmatic man of letters who received a record advance of £625,000 for his biography of George Bernard Shaw. ‘He’s coming to dinner tonight, tomorrow I’ll spend alone…’

Drabble, 53, is an intriguing woman. ‘Occasionally I dream about being naked in the front row of the stalls. Sometimes I’m falling off a cliff or missing an aeroplane,’ she says, cupping her face in her hand. ‘I have a recurrent dream of standing up to give a lecture and I don’t know what I’m talking about.’

She also has a famously frosty relationship with her elder sister, AS Byatt, the Booker prize-winning novelist and critic. There have been periods of potent rivalry when they haven’t spoken, and they refuse to read each other’s books. ‘I was,’ says Drabble, ‘terribly upset when she became very difficult about me.’ Normally exceedingly reluctant to discuss their relationship, she talks later at length about one of the literary world’s most famous tiffs.

Drabble became famous 30 years ago, aged just 23, with A Summer Bird Cage. Once compared to George Eliot, she is currently writing a biography of Angus Wilson, has written novels, screenplays, biographies and edited The Oxford Companion to English Literature. She created a new kind of domestic novel but now writes with ‘black humour about post-industrial society’. Her recent work had critical drubbings. ‘I’ve been around too long and was doing too nicely and they don’t like that.’

She is wearing a sensible black suit, brown shoes and greyish tights and has large penetrating blue eyes, a clear and unmade-up face and brown bob. She is equanimous with an outer confidence, a face that expresses little emotion and a plummy middle-class accent with no trace of her Sheffield roots. We are talking in her Edwardian redbrick house, in a sitting room with one white wall and the rest in 20-year-old William Morris wallpaper, with books everywhere.

We return to the subject of marriage. Drabble and Holroyd spent most of their courtship behind the wheel of a car – Holroyd allegedly teaching her to drive. Why do they have such an unconventional relationship? ‘Both of us absolutely love being alone and we enjoy one another’s company enormously. So we’ve struck up a way of life which allows us to do both. ‘I didn’t want a relationship that was threatening or confining. He didn’t expect me to change or alter my work pattern. So many relationships have a bad effect on both parties. You see them getting nastier or cosier – cosiness is bad, too. For us, every week is different. This week we’re apart two nights. But we can always ring and say, ‘I want to be alone tonight,’ or, ‘Why don’t you come to supper?”

The last of her three children (she has two sons and a daughter) left home last year, so she feels there is no further need for her house. She wants to finish off Angus Wilson and then move into Holroyd’s place next year.

‘We’ll carry on just the same. I absolutely have to have my own space. We’d have to have separate bits of the house. I’ll say things like, ‘I’d like to spend this evening on my own with a whisky and an omelette.’ ‘I think marriage is a dangerous, dangerous relationship, full of potential destruction of personality. People aren’t meant to be together 24 hours a day. The form of relationship we have seems much more natural.’ Are they faithful to each other? ‘It’s not a question that I would talk about,’ she says, sounding strained.

Sex is something that she regards as destructive as well as agreeable. ‘It can be consuming and obsessional and make you very unhappy. I think it’s an extremely important part of life that can cause a great deal of misery. Sexual jealousy is a terrifying feeling. Sex has caused me misery at certain times in my life. I was unnaturally nervous about it when I was young, and then terribly keen to get on with it when I was 18.’

Drabble’s father was a circuit judge who was frequently away and remote, and her mother an English teacher who was aggressive and difficult. The second child of four, she went to a Quaker school in York, read English at Cambridge and got a double first, acted with the RSC for two seasons and then started writing. Marriage to actor Clive Swift lasted 15 years. ‘I was quite fiery and would lose my temper as a child. My mother and father were depressives and we all got depressed. I had quite serious bouts of depression. I used to cry a lot. My mother had a really foul temper and was always telling me off. You just weren’t allowed to make mistakes. She was from a working-class background, so you had to get on in life. ‘I had a most terrible stammer as a child. I could hardly speak. It comes back when I’m tired or cross.’ It also seems to happen when conversation hits an emotional nerve.

So what of her relationship with her sister? ‘She’s written about how she hated me, and I’ve written about how I admired her,’ replies Drabble, her voice rising. ‘When we were little, we had just an ordinary sibling irritation. But things got worse when we became published writers. ‘I used to tag around after Sue (as she calls her) when we were children. I wanted to do everything she wanted to do, which must have been maddening. I published first, when I was very young – which must have been terribly annoying for her.

‘We both became wary of reading one another’s work, worried that we were using the same kind of material. I forced myself to read one volume of her short stories which, unbeknown to me, came up for an award. I found it an unpleasant and disturbing experience reading about my own father.’ How did she feel when AS Byatt won the Booker Prize? ‘I was delighted for her – I don’t let my books go in for it, but of course I felt a twinge of envy…

‘We get on much better now than we did. She had this tragic period in her life after her son was killed. But now for the first time in many years she’s actually enjoying life.’

What about the time when they refused to speak? ‘It wasn’t as obvious as that,’ she stammers. ‘We just didn’t go out of our way to see each other.’ Does she feel animosity towards her? ‘No. I feel flattered and honoured if she calls me. I’m still the little sister. I feel it is wrong to bother her. I’m actually quite frightened of her because she is older and cleverer than me.’

As Byatt is certainly older; but Drabble is extremely clever. She also appears dour, but laughs. And wise, enquiring, sometimes priggish but likable. ‘I suffer from doubt most of the time. Doubt of myself as a writer, cook, housewife and human being,’ she says, steadily. ‘I feel flawed and faulty as a human being.’