Penelope Fitzgerald is not the lucky kind of writer to whom subjects come naturally, headon, without ambiguity. Rather, they crop up unexpectedly, sneak up on her out of other matters, arrive when least expected. Innocence, published this week, might never have come to her at all had she not decided to spend a few spring weeks in Florence, with the idea of identifying the flowers in Botticelli's Primavera, and found herself instead absorbed in the marital squabbling of a contessa with whom she was lodging and her doctor husband from the south of Italy.


The flowers turned out disappointing: Botticelli had left them to assistants with no keen eye for botany - though the absence from the painting of the wild iris, now to be found all over the place, made her speculate, with a true scholar's curiosity, about the date it was introduced to Italy - and she discovered that the university gardens, supposed to contain an example of every Tuscan plant, had been given over to vegetables instead. However, the contessa's quarrels provided her with another sort of thread, and Innocence came to be written about 'people who don't fit too well - as many don't, I suppose'.


Though convincingly Italian in feeling, Innocence is not based on detailed research, over-attention to such matters. 'I don't think novels are about information', she says. 'If you wanted to know about Florence, you'd read a guide book. ' She was more worried about getting the Italians right, as people, not comic characters with funny accents.


Penelope Fitzgerald is one of those rare people who discovers a real talent only when well into middle age. In the Fifties she helped edit a literary magazine called World Review, but it was not until her husband fell ill 10 years ago that she thought to entertain him by writing 'what, in my opinion, men most like reading: thrillers and history'. The first two books were a biography of Burne-Jones - whose red and pink glass windows at Birmingham Cathedral were the first things in her life that had struck her as beautiful - and what she insists on calling a 'mystery', as if the word 'thriller' were to give it too much dignity, centred around the Tutankhamun exhibition, which she has always suspected was made up not of original objects but of fakes. Thinking she stood more chance with a publisher not known for its crime list, she took it to Duckworth, who had not got one, but who accepted her book.


Then she moved towards straight fiction. 'In spite of being so old and of such a literary family, I was very green. I didn't know you were supposed to write five thrillers before readers knew you. Anyway, I couldn't think of four more. '


Among the literary family was her father, E. G. V. Knox, editor of Punch, and the Catholic priest and writer Ronald Knox, and later she turned to a biography of the family. She wonders now why literature did not seem obvious to her earlier, instead of a somewhat haphazard progression from Somerville College to wartime work in the Ministry of Food and then the BBC. After the war, married and soon mother of three children, she stayed at home, living at Chelsea Reach on a houseboat until it sank.


In 1979, Penelope Fitzgerald won the Booker Prize for fiction, with her second novel, Offshore. It has altered her life considerably. It was the year the prize money reached pounds 10,000, awarded free of tax, and though she was embarrassed to find herself lined up in a row at the prize-giving as if still at school, with Kingsley Amis in the queue nearby, it has made her life as a novelist more possible. But she has not given up the coaching at Westminster Tutors, to which she says she is addicted: 'Perhaps I ought to stop. I'm an impostor, you know. I have no certificate. Anyway, I'm like wine in a bottle: I think I'm deteriorating. '


About her plans and about the future she is, as on all topics, modest. Penelope Fitzgerald has that endearing combination of extreme self-deprecation and the natural sharpness of someone whose entire life has revolved around intelligence and the use of the mind. She has just completed a number of introductions for Virago books and says that, while she pictures other writers dashing theirs off between coming back from the theatre and going to bed, she takes ages to do hers and worries incessantly about whether they are good enough. A plan to write a biography of L. B. Hartley, who was a friend, may be abandoned as may all biography, which she says has become alarmingly competitive.


What there will be, though, is another novel. To get going, she needs a title, a first paragraph and a feeling about how the book will end. After that, it is endless work, on old envelopes, losing bits, enjoying best of all the dialogue, which she sees as the moment in a novel when 'you feel close to the people and hear their voices'. Not, however, conversation, which she finds hard, and for which she admires Lawrence, who made it sound easy to do.


Penelope Fitzgerald divides her time between three rooms at the top of a friend's house near St John's Wood, with an old-fashioned gas-fire and postcards pinned to the walls, and her older daughter's house in Somerset. 'I don't really know where I live. It doesn't worry me. I know it's become immoral not to be busy, but I think I like pottering. ' In Somerset, she is in charge of the garden. 'Gardening, I think, is even worse than writing. There's all that worry about things not being out and vegetables not doing what they ought to do. '

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