'It's been a very volatile relationship. We've quarrelled our way through 30 years.' But the marriage (to Hunter Davies) has never been threatened? 'Never, no. It's not a case of resisting temptation - there's never been temptation, so I can't claim any credit.'
Margaret Forster and her husband have written 60 books between them in the past 30 years; at the same time he has been a journalist and she, famously, has been a housewife and mother. That she is houseproud is obvious as we walk through to her spotless dining room, the sun streaming in to highlight the polish on a typically Sixties Ercol dining table and a blue bowl of lemons in its centre.
She talks enthusiastically about her three-year obsession with Daphne du Maurier, whose biography she has written. It is a marvellous book. The private life of the author of Rebecca turned out to be a biographer's dream, and Forster has made the most of her material. Yet at the last moment, with the book virtually complete, she received dramatic new evidence.
'I had written early on to the daughter of Daphne's American publisher asking for letters and she sent some. They were nothing remarkable, but I kept writing to her, kept the ball rolling, until she came over to England and we had lunch and she put me through my paces. Then she said she had hundreds of letters which showed not only that Daphne had been in love with her publisher's wife, but that she had had physical relationships with two other women.
'At this stage I'd finished the book, and it had all been cleared by Daphne's literary executors, so I then had three absolutely horrendous months last spring in which I rewrote the whole heart of the book, and made other adjustments all the way through.'
Did the revelation that du Maurier had had lesbian relationships surprise her biographer? 'I had instinctively suspected but hadn't hitherto been able to spell it out.'
But first, Margaret Forster's own life. She has lived with her husband in this same north London house since their marriage. Both came from respectable working-class families in Carlisle; both attended good local state schools; both went on to university: in Margaret's case, Somerville College, Oxford. 'I got married in 1960, in my last week at Oxford. In the working-class culture we came from, not to be married was a source of embarrassment and shame. When we lived together in the long vacation we had to conceal it from our parents.
'I started as a writer almost at once. My first novel, Dame's Delight, was published in 1964, followed a year later by Georgy Girl. It was so easy to get published in the Sixties] Publishers won't take risks on first novelists now, but then they expected to lose money on your first two or three books. Margaret Drabble and Sheila MacLeod were my exact contemporaries.'
Despite belonging, by age, achievement and fashionable north London location, to a group of influential writers and critics, Margaret Forster has never really joined. She hates big literary events, declined a launch party for the du Maurier biography, and is, in her own words, a housewife and a loner.
She is unusual in having brought up three children and run her house without any help, while pursuing a successful career as a prolific author. She sighs. 'I'm not unique. I'm only unusual in the media world - the London world. Babies are tough to look after, and I look back on those early years as a complete nightmare and think, how on earth did I do it? But as to why I did it without help: I couldn't bear the employer/servant relationship and dreaded having to share my privacy.
'I love being in charge of my own house. I love decorating it, looking after it - I wouldn't move, though it's just an ordinary house. This is where my roots are. The children have literally been born here, we've dug into the neighbourhood, and I need that stability.'
Such traditional views are anathema to feminists, but Forster speaks only for herself. 'The strain and stress of 'having it all' is colossal, though I do think women should have it all. But personally I couldn't have coped.'
Does she have much in common with Daphne du Maurier? 'Not very much. We obviously come from completely different backgrounds, except that we're both genuinely antisocial: that's the biggest bond. I loathe socialising and I don't do it; she hated it, and had to do it. The great difference between me and her was motherhood and the domestic front. Daphne was very fond of saying she'd never lifted an iron in her life.
'I love Daphne's self-awareness, her mocking style, the light-hearted side of her (very difficult to capture), her exquisite manners. I like her modesty as well - she had no high opinion of herself and never realised quite how famous she'd become. She was a tremendously hard worker - it's no game, writing - and I admire the hours she put in, going out to that freezing little hut in the garden.'
And the new evidence: did the revelation that she had had lesbian relationships surprise or distress Daphne du Maurier's three children (who are all still alive)?
'They say they didn't know anything about her relationship with Gertie (Gertrude Lawrence, who starred in a play by Daphne du Maurier); but they were adolescents at the time, and their mother never missed an opportunity to refer to Gertie and Ellen (her publisher's wife) in a most dismissive fashion. Indeed, she was disparaging about gays in general. I take her at her word when she says she was not a lesbian but, in her own expression, a 'half- breed': drawn at times to both sexes. I begin to wonder if that's so rare.
'She was definitely physically attracted to her husband when she married him, but at other times in her life she needed sexual affection from both men and women. Of the two she preferred 'Venice' - as she called the love of the same sex - to 'Cairo', her code word for heterosexual sex.'
Daphne du Maurier was her father Gerald's favourite child, although he had longed for a son. She always felt there was 'a boy in a box' within her. Does Forster feel this upbringing contributed to Daphne's lesbian proclivity? 'It wasn't her father who 'made' her lesbian: she was born that way. I think her husband suspected, but he could never challenge her because he didn't want to admit it.
'In any case, after the war ended and he came back they never talked intimately. The war damaged a lot of marriages. For six years you didn't see your man except for 48-hour leaves - and the men came back strangers. In their day, pride and control meant you never confessed. There was a stigma about a failed marriage, a divorce, and people's behaviour was governed by this: you made your marriage work.'
What about Menabilly, Daphne du Maurier's beloved home in Cornwall . . . was it the origin of 'Manderley' in Rebecca?
'The feeling for Manderley, its setting and atmosphere, are hers for Menabilly, but the actual house described in Rebecca is Milton, a house in which she'd stayed as a child. Menabilly was only rented, albeit for nearly 30 years. She believed she had come to own it through right of love.'
Why did Daphne du Maurier feel the need to write? Most women would have been content with being the mother of three children and wife of 'Boy' Browning, Comptroller of Princess Elizabeth's household.
'Writing was her life and without it what she called her 'Number 2 Persona' - her interior self - caused trouble. He (she always called this inner self he) was responsible for the violence and viciousness in her, for the nasty horrible short stories in which people were murdered and had their throats cut. Writing was essential to her sanity.'
What will Margaret Forster write next, so that her own 'Number 2 persona' can escape?
'It'll be fiction. The worry of biography is crucifying - the weight of responsibility - whereas fiction is gloriously free, so I shall sit down and write a novel. I keep thinking about victims and their families: all those people whose lives are completely wrecked.'
Behind that flawless facade, the unconscious mind is at work like the mind of Daphne du Maurier, delving into the wicked, the sinister and the terrifying. Writing is what enables Margaret Forster to be a sane and happy housewife.
'Daphne du Maurier' by Margaret Forster is published by Chatto & Windus at pounds 17.99.