Perhaps that is why Dorothy L Sayers chose to open her final, unfinished detective novel with one of those classic set-pieces in which assumptions about the central characters are set up and promptly demolished. Harriet and Peter, according to someone dining in the same restaurant in Paris, look like "the English married couple par excellence" - polite, bored with each other, lacking in passion. On the contrary, says his companion, who happens to be Peter's uncle. Wimsey fell in love with Harriet as soon as he met her pursued her for five years, and they are even now returning from a prolonged honeymoon. "Mayfair," he adds, "is awaiting the result of this curious matrimonial experiment."
In the same restaurant is another English couple: Laurence Harwell and his stunningly beautiful wife, Rosamund, whom he rescued from penury after her father went to prison for fraud. The scene is thus set for a portrait of not one but two marriages, one conventional and the other unusual, and the contrast between them is made all the more stark when one of the Harwells is murdered. These events take place against the background of the death of the old king, George V, and the accession of a new one, Edward VIII, with rumours already emerging from France of his affair with a married woman.
In that sense, Thrones, Dominations, which is Sayers's own choice of title, is an ambitious portrait of a nation in the throes of modernisation as well as of the internal politics of two marriages. This is a heavy weight for a detective novel to bear, and may be one of the reasons why Sayers eventually abandoned the book in 1936, leaving an uncompleted manuscript and notes. The novel has now been finished by the author Jill Paton Walsh - whose own novel Knowledge of Angels was shortlisted for the Booker Prize - an enterprise which inevitably raises questions about the wisdom of posthumous publication and the ability of one writer to stand in for another.
There are many novels whose characters exert such a grip on readers' imaginations that the longing for sequels is not extinguished with the demise of their authors. Authors as diverse as Jane Austen and Margaret Mitchell have attracted novelists who have completed unfinished manuscripts, as in this case, or written so-called sequels from scratch. Hodder & Stoughton, the publisher of this version of Thrones, Dominations, say that Paton Walsh has finished the text "in Sayers's voice", which is of course a matter of opinion. To be fair, there is no obvious moment in the novel when Sayers stops and Paton Walsh takes over, and yet the overall atmosphere, with its close attention to period detail and its archness of tone, is near to pastiche.
The treatment of the Wimsey marriage, with its high-flown love-talk and endless bouts of soul-searching, suggests that neither novelist is able to imagine the characters as husband and wife, which is why the Harwells' troubled union plays the role of useful diversion. But this shifts attention to the murder plot, which is in itself so stuffed with red herrings as to offer another possible explanation for Sayers's decision to abandon it. The conventions of 1930s crime writing have never had much relation to realism but even they are stretched by the absence of any credible alternative to the murderer, while a series of coincidences and some frankly silly business with a papier-mache mask reduces the denouement almost to farce.
The question which remains is why, 50 years later, anyone would want to take on such a project. Sayers, who died in 1957, had plenty of time to go back and finish Thrones, Dominations but decided - wisely, I think - against it. The fact that Paton Walsh was educated at Oxford, like Sayers, and taught by Sayers's friend C S Lewis - these connections are cited by the publisher in publicity material - do not explain why she wanted to embark on what amounts to a literary impersonation. Of course, finishing another writer's novel may be the ultimate homage of a dedicated fan but the result is likely to be an unsatisfactory hybrid, prompting unanswerable questions in readers' minds about both plot and characters.
In a sense, what Paton Walsh has done is remind us of the shortcomings of the Golden Age detective novel without displaying its strengths. The atmosphere of a genuine Sayers novel, or indeed the books Agatha Christie wrote around this time, convey a powerful sense of a social fabric strained and torn by outside events, such as the First World War and the Bolshevik Revolution. Their characters struggle with forces they do not completely comprehend, creating a sense of urgency which is replaced, in Thrones, Dominations, by a reliance on hindsight.
What the book proves, in the end, is the uniqueness of each writer's vision. It is possible to look up details about train times and postal deliveries, to know the earlier novels inside out, and still come up with something lifeless. Among the papers Sayers left behind, according to a letter she wrote about the progress of Thrones, Dominations before she abandoned it, was an outline of the plot drawn in coloured inks. Jill Paton Walsh has filled out the features of this map, adding local colour: but it remains a map nonetheless.