NICHOLAS MOSLEY, novelist, alias Lord Ravensdale, lives in the basement of a large house on the edge of Regent's Park. His children (five by two marriages, the youngest still in his teens) have colonised the top three floors. So as to work in peace, their father has his study at the lowest level, in what must once have been the servants' quarters. His second wife, Verity, a psychotherapist, has a room next door to his, but 'as soon as you go upstairs, you're in the world of children'. One cannot imagine his father, Sir Oswald Mosley, being equally considerate to the needs of his offspring.

The study in which he writes could be that of a clever, rather chaotic undergraduate. Into this L-shaped room are crammed a bed, a couple of bookcases, several more feet of shelves, a large desk littered with paper, two armchairs, a one-bar electric fire and an eclectic mixture of pictures and ornaments, including several bronzes of Shiva encircled by fire.

Nicholas Mosley has published 12 novels, the latest of which, Hopeful Monsters, won the 1990 Whitbread Prize. Few have been bestsellers; most are well reviewed and all are now being reprinted in paperback. His publishers, Minerva, expect them to sell around 6,000 copies each: a respectable number for a 'literary', meaning serious, novelist.

Nicholas Mosley is 70 this year but looks younger, being tall and lean and upright. With age, he has come to look disconcertingly like his father. He has the same hawk nose, high forehead and commanding gaze. Well, perhaps not quite the same commanding gaze - Oswald Mosley's eyes cut through a crowd like a laser.

Married first to Lady Cynthia Curzon (Nicholas's mother) and then, after a long affair, to Diana, the most beautiful of the Mitford sisters, Nicholas Mosley's father was the most charismatic politician to emerge between the two world wars. During the Thirties, his British Union of Fascists, known as the Blackshirts, caused riots when they tried to march through the East End of London. A demagogue and associate of Hitler, Oswald Mosley must have been an appallingly difficult man to have as a father, especially for a clever, highly strung, slight, stammering boy like young Nicholas. Yet his elder son has made his name as one of our most original novelists, and has also written a life of his father that balances filial insight with scrupulous regard for the facts.

'I had always intended to write about my father because I felt I had something to say which would take him out of the myth of being either a monster or a hero. I wanted to describe this immensely interesting human being with a life-giving force which sometimes tipped over into a demonic exultation in his own powers and talents.

'Just before he died - two weeks before, literally - I had been over to Paris with my second wife, Verity, to stay with him. He was very old and mellow by then and we talked about old times. I said, 'Look, Dad, someone must write a book about you: not to justify your politics but because you're such an interesting human being.' The next day at lunch he said to my stepmother, Diana: 'I've been talking to Nick and I'd like him to have all my papers.'

'And then he died; and, after an interval, out of the cellars at one of the Devonshire family houses at Lismore came this fantastic hoard of papers with sacks of letters from my mother's early life, including letters from George Curzon, (her father, who had been Viceroy of India and Foreign Secretary) and all about my parents' marriage. Diana very nobly said: 'OK, Nicky, you take all these]'

'I learned a lot. Not many new facts, because I'd been told endlessly by my aunts and kindly friends what a difficult marriage my parents had had, but I hadn't known my mother's side of the story before. She emerged from her letters as a wonderful character, so I wanted to save her from the limbo of being just a doormat.

'My father used to say: 'I sometimes think I'd have done much better in politics if I hadn't spent so much time running after women.' I don't think it was a power game for him. Any beautiful woman, if she was available, he wanted to sort of 'romp'. He must have been very highly sexed. I'd always known that he was supposed to be the most monstrous philanderer, and it seemed stupid to take a high moral line. I couldn't understand all those reviewers who suddenly got on their high horses about it: many of them people who, over the last 30 years, had done quite a bit of 'running after' themselves.'

Sir Oswald once described his relationship with his eldest son as 'ideal'. Would Nicholas Mosley agree that his childhood was happy?

'From a very early age I went into a world of my own and found a tremendous circle of friends. My sister Vivien had terrific girlfriends from a very early age and I had school friends and I went to stay with them in the holidays and they with me and we got huge happiness from our boys' world. I suppose I used to get more depressed than would be natural, but I just thought it was ordinary. It would never have entered my head that I was having an unhappy childhood.

'My father never tried to influence any of us about politics, absolutely not at all. He was the most marvellous person to talk with when I became about 15, 16 - about history, literature, ideas.'

Today, Nicholas Mosley wears beige cotton trousers and a navy blue shirt, both with that faded English elegance that Ralph Lauren would die for: though he has probably never heard of Ralph Lauren. Despite the stammer that has never left him, he is articulate and expressive, speaking in complete sentences illuminated by exactly the right word.

'I never thought I would do anything other than be a writer. The war started when I was 16. I knew I would have to go into the Army when I was 18, and I thought I would be lucky not to get killed.

'In fact, I was 23 by the time I got out of the Army. From the age of 21 I had had a private income from a trust through my mother's family. After the war it was about pounds 2,700, which would be more like pounds 36,000 today.

'I married in 1947. My wife, Rosemary, and I wanted to be the new generation and we certainly wanted to be faithful. The convention of my parents' generation was that you should not have an affair with an unmarried woman; you should only go to bed with your friends' wives . . . which seemed absolutely ludicrous. We planned this idyllic, cut-off life in a Welsh farmhouse, which never really got off the ground. When we married we were very mixed-up and I think we got some things wrong with our own children. We were still involved in the residue of nannies and the old life.

'The conventions of marriage until the Fifties were based on the belief that men had strong sex drives and women needed romantic love. But sex was something their husbands had to have. I think a lot of women genuinely didn't much enjoy sex, and what altered that we don't really know, for all the sociology of sex and feminism. Until D H Lawrence, nobody had ever talked about the female orgasm or the fact that the man had a duty to try and give the woman an orgasm - this was something an awful lot of women hadn't known about.

'My first marriage lasted from 1947-1973, so 25 years, and for about 17 or 18 years of that it was a good marriage. Rosemary was a painter and she wanted to do that more and more, and I was writing books and scripts, which took me away and into this funny film world where you're always hanging around. In this condition a lot of other complications crop up.

'But back in the early Fifties I was a fairly representative young writer: agnostic, and, I suppose, confused about the world and how one behaved . . . all the paradoxes of trying to be married and a good family man and find out how life worked; and one got into various predicaments that were quite difficult to deal with.' Having published his first three novels - 'very Angry Young Man, all gloom and doom' - and a travel book, Mosley met a remarkable man called Father Raynes, who was head of the High Church Community of Resurrection in Yorkshire.

'Father Raynes, one did seem to think, was a holy man. It is hard to put one's finger on it, but there was some transparency through which truth or authority came, and I was enormously impressed by him. In the presence of Father Raynes I found myself saying, 'You know, I'm not so very awful' - by which I meant that I recognised I was in a fair old mess, though I didn't like to admit it to anyone else. He died in 1958 after I'd known him for five years, and the community asked me to write his biography.

'By the end of the Fifties, I decided to make some commitment to being a Christian, rather than just arguing about it, so I became churchwarden in our local Sussex village and did the disciplined things. I wrote no novels for some years; instead, I wrote this biography and for two years edited a little Christian magazine called Prism.

'After a few years I felt very strongly that I wanted to write novels again but in a quite different style. The earlier novels seemed self-indulgent and pessimistic. In my middle-period novels - Meeting Place, Accident, Impossible Object - I stopped being a spoiled, rich kid, began to be appreciated by readers and hit the big time commercially.

'But then suddenly my life came to a dead end. The films I'd written scripts for weren't a success. The next novel got a hatchet-job review in 1970, so rather to mark time I wrote a biography of Julian Grenfell (a First World War poet), and then embarked on this incredibly obscure work called Catastrophe Practice. I wanted to find a new way of saying we all do bad and stupid things and hurt other people and ourselves, but either things are OK or not.

'One has to work out how to reconcile what one learns of the world with one's religion. I became obsessed with trying to do that, and I've been trying ever since. I felt that at heart, the Christian message was that in spite of all this . . ' (he gestures, and I understand that he means the confusion, inadequacy, temptation, pain and self-doubt) 'life is OK. So although I don't do all that many churchy things, the whole Christian pattern and story and imagery and message has meant an enormous amount to me, because I think it actually works.

'I thought and talked and read a lot about it, including reading the Bible straight through, and this was a great eye-opener. In the Old Testament people were looking for rules and regulations by which to live and the failure of this attempt leads to the New Testament, in which, by admitting that you can't live by the rules, you need faith, you're saved.

'Then, having learned all this, you're on your own. You will have found something outside yourself that the Bible calls the Holy Spirit: 'Watch and listen and you are led into the way of all truth'.'

We had talked for hours. Dusk was falling. I listened - as he perhaps, 40 years ago, had listened to Father Raynes - with the sense that more was being said in the undertow of words than on their surface.

'It's easy to think one can make something positive out of great tragedy and pain. I wanted to say that through looking at the painful and squalid things as they are, one learns the extraordinary pattern of life . . . that it renews itself. If you have the courage to look at what's going on and not justify yourself or put the blame on other people, then in some curious way life helps you to get on to the next stage - not a cool, calm plateau but probably a harder and harder stage.

'If one faces up to what's happening truthfully, then it seems to me that by some extraordinary almost miracle one has learned more than if one had gone on trying to stick to conventional morality and tying oneself into knots. This, it seems to me, is exactly what Christ was saying: don't sin, but if you do sin and face it, you're probably better off than the people who never sin. To be human is to sin, but you still have to say, 'I'll do my best not to.' You have to take responsibility for what you actually are and not what you would like to pretend you are.

'You may go through pretty rough times, but in the end you're probably wiser than someone who thinks it's all there in the rules. Something has been born inside one, almost secretly, which says: you can do it by yourself. That's what life is, that's what being human is, yes.'

Minerva will republish Nicholas Mosley's 1966 novel, 'Assassins', on 26 July.

(Photograph omitted)