A mystery to baffle even Miss Marple

To bolster his thesis, Marr made the amusing observation that Boris Johnson is a modernday Marple. Both come across as amiable and bungling and play the fool to great effect. But underneath their "ruthless, coldeyed English cosiness" they are actually very clever.

Whatever the merits of this theory, it doesn't really explain Christie's phenomenal success and why she continues to outsell Shakespeare. The Bard is a much better brand and a much better writer, whereas Christie's novels are not even particularly well written.

PD James believes that Miss Marple's emergence as the lady detective par excellence depended on Joan Hickson's portrayal of the character in the TV series. However, it can't all be down to television, as Miss Marple was popular before the 1980s.

Surely it is the simplicity of her tales that explain her universal appeal. Her stories are not overwritten as are, say, Dorothy Sayers'. As a result they haven't dated in the same way. And her characters are all instantly recognisable stereotypes. Add to this a cunning plot and you have the recipe for enduring success.

In fact some of her stories are so artfully contrived, it's a miracle nobody thought of them before.

Christie gave us the plot where all the suspects did it (described by Raymond Chandler as so ingenious only an idiot would guess it), the plot where the narrator did it, and the plot where the policeman did it.

Years ago I went to an Agatha Christie conference in Torquay. The attendees were true believers and far from being a celebration of Englishness, I found the whole occasion rather sinister and cultish. Then, to my horror, my wife joined the cult. The opening of Nemesis inspired her to start writing a detective novel. Read it and you'll see why. It is a chilling account of a woman on the brink of senility.

Agatha Christie's stories are at once unsettling and reassuring.

The villain is always discovered, loose ends are tied up and everyone returns to their cosy English idyll. Christie came to dislike her character Hercule Poirot, as she found him smug and annoying, but Miss Marple remained a favourite creation. To judge by her continued sales, the rest of us agree.