Dorothy L Sayers left a mystery novel tantalisingly incomplete.
We were walking in Borrowdale when my agent Bruce Hunter told me, apparently casually, that an unfinished novel by Dorothy L Sayers had turned up in an office clear-out, and that her literary executors were looking for an author to finish it. "Wonderful!" I said. "Absolute treasure for somebody!" I had no idea at that moment that the fortunate somebody would be me.

The treasure in question consists of six chapters of a Peter Wimsey detective novel, and a mysterious plot diagram. The title is Thrones, Dominations, a quotation from Paradise Lost: "Thrones, Dominations, Princedoms, Virtues, Powers / If these Magnific Titles yet remain / Not merely titular ..."

The inheritance or renunciation of titles was clearly one of the themes of the book. It was set early in 1936; late in 1936 the king abdicated to marry Mrs Simpson. Sayers abandoned Thrones, Dominations, that same year, although Victor Gollancz, her publishers, were still announcing it as a forthcoming title as late as 1939. Now the treasure is mine; but it remains, of course, hers. I am fascinated, daunted and exhilarated by the task - fascinated by the technicalities, daunted by the difficulties, exhilarated by the prospective fun.

The reason, if it can be recovered, why Dorothy Sayers abandoned the novel, and the propriety of finishing it for her, are closely linked. Her correspondence for 1936 shows her to have been desperately busy. She was struggling with Busman's Honeymoon - the play - then in production, and with the novelisation of the play. She was in the first stages of a long and triumphant love affair with the theatre. Her enthusiasm was to lead her into Christian playwriting, and then to her culminating achievement, the translation of Dante. She was so hard-pressed there is no need to look further for a reason for the suspension of work on Thrones, Dominations, however tempting other theories may be. It is easy to see how she might have pushed Wimsey aside. Yet he did not die on her - he continued to figure in her mind, appearing in a series of articles for the Spectator during the war, and in later short stories.

I was afraid that there might have been some terminal glitch in the conception of Thrones, Dominations which made it unfinishable; but I can find no such thing. Writing to her friend Helen Simpson, Dorothy Sayers expressed satisfaction with the plan for the novel, and she seems to have been right. We have a good cast of characters, and a backdrop of London theatre life; we are watching Lord Peter Wimsey and his new wife Harriet Vane settle into early married life - surely the murder must be about to happen - ?

That is for me to work out, and working it out is a detective story in its own right. The first thing is to assemble every detail which might illuminate what Dorothy Sayers had in mind. The trail leads to her papers, deposited in the Marian E Wade Center in the library of Wheaton College, Illinois, into the archives of the Dorothy L Sayers Society, and to her letters, edited by her friend Barbara Reynolds. The first volume of these, already published, brings us just up to the point at which Thrones, Dominations was abandoned. Sayers enthusiasts on both sides of the Atlantic are friendly and eager to help. There is an extensive secondary literature, in books and in journals published by the Wade Center in the States, and the Sayers Society in Britain. Everything I could want to know about Lord Peter Wimsey and his circle has been annotated by a devotee already. As to her theory of detective fiction, Dorothy Sayers is on record: "What nearly always happened to Peter is to collect all kinds of facts, usually in a mood of hopeless bewilderment, until quite suddenly, the essential facts of the HOW arrange themselves to form a synthesis (by which time, of course, they usually include the WHY as well)."

All this is enormously helpful. But when the collection of known facts is accomplished, I shall be on my own, inventing on someone else's behalf. I have never completed another author's work before, and I have mixed feelings about it in general. We are seeing an outpouring of sequels at the moment, some of them rather cheeky. But a completion is a different thing from a sequel. It isn't an unnecessary extension of someone else's work; it is necessary to allow the fragment to see the light. Not so much like a fake Grecian urn as one of those museum pieces in which a surviving shard is mounted in a reconstruction, so that you can see what the real bit was meant to be. Just as the completion of Mozart's Requiem allows its satisfactory performance, so the completion of a fragment of novel should allow it to be a satisfactory good read. A fragment in a non-viable state is otherwise of interest only to specialists.

Would Dorothy L Sayers have been horrified? She was not likely to have minded collaboration as such. Of all the writers I have ever read about she was the most inclined to share her work with her friends, to discuss work in progress, to invite help and comment, to play literary games with correspondents - like the joyful and elaborate exchanges in which she and C W Scott-Giles, a keen genealogist, provided Wimsey with a family tree. She collaborated with a scientist, Dr Eustace Barton, who had provided the ingenious scientific background for The Documents in the Case, and generously shared the credit, though the writing was entirely hers. Even when Peter Wimsey was involved she could bear to collaborate: the play Busman's Honeymoon was co-authored with Muriel St Clair Byrne, an old college friend who contributed both the necessary understanding of dramatic art, and endless encouragement. Dorothy Sayers would not, I think, have disliked the mere idea of a dish with two cooks.

Of course, if she were still around to choose her own collaborator, she might not have chosen me. But 60 years on it is now rather a question of my choosing her; for there are few authors in whose voice I could undertake to write. I would need a certain amount of admiration, of sympathy, of liking, of ground in common, before I could attempt it. And I do feel the admiration, the sympathy and the liking. I do not, of course, share all Dorothy L Sayers's attitudes, although I partly share her background, being an Oxford graduate with a High Church, in my case a Roman Catholic, upbringing. But crucially I am, like her, a pre-feminist blue-stocking woman. The struggles that confronted all such as we, trying to reconcile a hunger for the life of the mind with the stereotypical demands of traditional women's roles and the normal human needs for happiness, have given me an instinct of empathy for her lifelong belligerence on the subject.

Some of what I have in common with her was actually her doing; for had I not read Gaudy Night at an impressionable age, I am not sure I would have worked as hard, as determinedly as I did, to get into Oxford myself. I have read an article which sniffily dismisses the picture of academic life and ideals offered in Gaudy Night as outdated and naive. If it is naive, it is certainly inspiring - I am one among many women whose aspirations were uplifted by it. If it is out of date, then so much the worse, I cannot help feeling, for Oxford.

And I can understand perfectly the fatal charm of Peter Wimsey for his creator as well as for his wife; I too, when young, was in love with him. Dorothy Sayers was notably unsuccessful with men in real life; in her imagination she made herself a hero fitter for the heart's desire. It isn't his wealth or his title that make him so attractive; it isn't the cleverness that empowers him as a detective; it is the desire, in a man who is in a position to choose, to have an intellectual equal as his partner. Peter isn't unique: there is Benedick in Much Ado About Nothing; there is Ralph Touchett in The Portrait of a Lady; there is something of the same flavour in Jane Eyre's Rochester, but few men in literature or in life have felt like that.

Well, in Gaudy Night Peter won his Harriet, and in Busman's Honeymoon possessed her, and in Thrones, Dominations we must begin to see how things worked out for them.

My first thought, contemplating the task that lies before me, was that I must know all that can be known about the Wimsey novel characters, but that I did not need to become an expert on Dorothy Sayers herself. I was wrong. For the author is always a secret character in a work of fiction. And the author-in-the-work is always an impersonation; it's just that this time I must impersonate not myself, but somebody else. I must think and work, read and dream that somebody else, until she becomes as clear to me as one of my own characters. Between us we must create the prospective author of Thrones, Dominations. I look forward to being haunted by her, and to being able to claim that I am Dorothy L Sayers's ghost.

! 'Thrones, Dominations' will be published by Hodder and Stoughton in Spring 1998.