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"Don't write about the boy who hears colours," advises the English master, in Alasdair Gray's hellish masterpiece of Glasgow gothic, Lanark. "Write about something more commonplace," he suggests, rejecting his young pupil's contribution to the school magazine. Trying to blend realism and fantasy, he explains, is best avoided, even by adults.

AS Byatt, Terry Pratchett (right) and Alasdair Gray have all chosen to disregard the sensible schoolmaster's advice. On Wednesday, at London's Queen Elizabeth Hall, the trio of writers read some of their weirder tales and explain why they enjoy letting their imaginations stroll freely across fantastical landscapes.

In the best feminist tradition, Byatt likes to play with fairytales, weaving contemporary allegories, constructing riddling mazes, or diving from the pre-Raphaelite ramparts into a swooning world of knights bewitched by serpent-tailed witches. Byatt also harbours an unexpectedly blokeish streak, it emerges, and is a huge fan of Terry Pratchett's Discworld - the Tolkeinesque flat planet which floats through space on the back of four elephants standing on a turtle.

Discworld is so real a place for many readers that Pratchett fans regularly sit down to fill in the gaps. Perhaps we'll find it was Byatt who knitted him the scarf for the Unseen University of Ankh-Morpork and started the official Mrs Widgery's Lodger morris-dancing team.

So, is there any real difference between constructing fictional locations like Hardy's Wessex and Trollope's Barsetshire, or Tolkein's Middle Earth and Alasdair Gray's disintegrating canibalistic dystopia Unthank?

"Well, according to Leavis' Great Tradition, stories should present a convincing social surface, let's call it the mode of the convincing lie," considers Gray. "The other option is fantasy, the mode of the tall story. It's left to the reader's judgement where one begins and the other ends."

Far from offering escapism, the best fantasy, is really a circuitous route by which to confront reality. "Think of Gulliver's Travels," says Gray, "where Jonathan Swift describes an island where people qualify for political office by walking a tightrope, and a prime minister is so adept he can turn somersaults on it. Children will say that's fun, but adults will say, Oh yes, that's Britain all right."

`Discworlds Apart': AS Byatt, Alasdair Gray and Terry Pratchett discuss the creation of imaginary worlds with Jenny Uglow, Queen Elizabeth Hall, South Bank, London SE1 (0171-960 4242) 23 Sept, 7.30pm