"I have been on a diet and lost three stones," he says proudly as the tailor looks pale. "There is a bit of tension under the arms, ja?"The production designer John Gunter watches. He loves Glyndebourne, but as a man who made his reputation in the subsidised sector, he has ambivalent feelings "I love to wander on the Downs It clears your head And there are so many skills here to call on But it took me time to like it Inevitably, you have the feeling of it being exclusive. You're taken in to be part of the family and it's like going back to school. But once you understand what they can give you here, it's wonderful.". Michael Frayn is a slippery character, a hoaxer In Frayn's world, you never know what's true or false.

His plays Noises Off and Clockwork showed people trapped in rapidly escalating farces; his current hit, Copenhagen, relates Heisenberg's Uncertainty Principle to the indeterminacy of human motive; and his 1999 novel, Headlong, deals with forgery and deceit Michael Frayn is a slippery character, a hoaxer In Frayn's world, you never know what's true or false. His plays Noises Off and Clockwork showed people trapped in rapidly escalating farces; his current hit, Copenhagen, relates Heisenberg's Uncertainty Principle to the indeterminacy of human motive; and his 1999 novel, Headlong, deals with forgery and deceit. So it's no surprise that his latest book, Celia's Secret (Faber & Faber, £12.99), is about a curious practical joke. Co-written with actor David Burke, it tells the story of how, during the run of Copenhagen, Frayn received letters and documents from Celia Rhys-Evans that seemed to shed new light on the nuclear physicists Werner Heisenberg and Niels Bohr, whose mysterious meeting in 1941 is the subject of the play. It soon emerges that these are fakes, forged by David Burke, the actor playing the good-natured Bohr.Organised in two acts, with alternating chapters by Frayn and Burke, Celia's Secret ostensibly shows Frayn coming to terms with his credulity and Burke revealing his own obsession with tricks. Like Copenhagen and Headlong, it pictures people as a mixture of high-mindedness and greed, and asks intricate questions about why they act the way they do. Reading it makes you feel insecure.So when Michael Frayn phones to postpone our interview, my suspicions are aroused.

Is this well-spoken, brisk voice Frayn's - or am I the victim of some prank? Such doubts vanish the minute I meet him His friendly manner gives no sign of the sinister spoofer. Only one detail hints at eccentricity: he answers the door in bare feet.Asked why he has published this embarrassing tale of being duped, he answers with typical modesty. "Well, I made such a colossal fool of myself, and so publicly, that there was no possibility of concealment. I also thought that David's hoax was so well perpetrated that it would make a good story."At the core of the hoax is a moral puzzle. When another actor confidentially tells Frayn he's being fooled, Frayn is unable to confront Burke without betraying the source of his information. Instead, he tries to hoax the hoaxer, and ends up in a farcical "I know that you know that I know" nightmare.

"I don't pretend I wasn't mortified," says Frayn, "but once I was in this situation, I could not get out: I had to keep up the pretence of believing even when I realised the truth."At first, "the hoax depended on my wanting so badly to believe in these documents that when they arrived, everything that was most preposterous about them counted in their favour". The first letter contained a document which, in semi-literate German, held instructions for putting up a table-tennis table. "If it had been about nuclear physics," he laughs, "I probably wouldn't have believed it."The point is not that seeing is believing, but that believing is seeing. "Once you've decided on an idea," he says, "you'll bend all facts to suit it." What made him so gullible? "Vanity: I'm not a linguist," he says, "but I'm very proud of my ability to read German." Also, in an echo of the protagonist of Headlong, "The hope of gain: I thought this discovery would make a nice little book."As Frayn talks, I admire his ability to tell this shaggy-dog story with such sincerity. You don't need a literature degree in EngLit to realise that, whatever else it is, Celia's Secret is obviously not an account of events that actually happened. The structure is too neat, with two acts each ending in a celebratory dinner.

And the two narrators speak in exactly the same linguistic register.Frayn is amused at my scepticism. "That's my job," he says, "I write fiction; I make up characters." But why expend so much effort on a prank? "I went to a terrible amount of trouble, but it was very enjoyable and it really focuses the mind." Besides the playfulness of the game, he's also making a "serious point about how people accept one interpretation and stick to it. Once they've made up their minds, few people ever stand back and dispassionately review the evidence."Once again, we're on philosophical ground. "How can we be sure that something has happened? We can't," says Frayn.

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