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His father’s endeavours paid for a place at Uppingham. “It was the school from which Stephen Fry ran away. There were still beatings and prefects and early-morning runs and cold showers.” Haddon was something of a maths prodigy, taking his O levels at 14, and A levels at 16, before switching to the arts and studying English at Oxford, where he rebelled against his background by reading The Female Eunuch and other feminist tomes, taking it all terribly seriously.

He is no longer so earnest. There is a glorious moment in Curious Incident when Christopher sees his mother’s worldly possessions thrown on to the front lawn. Two books are included: Jilly Cooper’s Riders, and Diana: Her Own True Story. Haddon and his wife own both. “When my wife switches off she has to read trash, so she’s got a few Jilly Coopers upstairs. Someone for a joke bought me Diana, which is not the greatest book, though it does contain a fantastic line. Prince Charles and Di were on a skiing holiday in Klosters when a friend of theirs was killed in a ski accident. Diana is described as gathering his possessions together after his death, ‘including his Al Jonson fun wig in which he had entertained the party the previous night’. The whole book was worth it just for that sentence,” he says with a chuckle.

He has an acute sense of the bizarre, which was sharpened by a spell, post-Oxford, working with adults who had learning difficulties. “I worked at what was then called an Adult Training Centre in London. It was a complete misnomer because they weren’t being trained for anything. Once the staff asked ‘the trainees’ what they’d like to be known as, and there was a wonderful man in his late fifties, who was articulate but had few other social skills. He said ‘I think we should be known as the obedient dogs’.”

Haddon is quietly impassioned about the way in which the mentally handicapped are kept apart from the rest of us. “We used to take the trainees to Mencap discos, where they weren’t allowed any close dancing, and we had to stand up for the National Anthem. In the end we boycotted them. They were a way of segregating these people.”

Ultimately, he decided that social work wasn’t for him, and took up illustration, working from the sixth floor of a Hackney tower block, and contributing drawings to New Statesman and The Sunday Telegraph. He met his wife at a party in Hornsey, North London, and “when the relationship really took off it was great because I had an excuse to move to Oxford”.

Though he enjoyed moderate success, Haddon wanted more. “I felt for many years as if I’d got my face pressed to the cold windowpane of the house of literature saying ‘Let me in!’ Granta’s Young British Novelists was a particular torture. I distinctly remember reading not the last one, but the two before, and thinking ‘I would so love to be in there. That is my ambition’. I felt this deep ache flipping through it. I was too old for the last one,” he laughs. “But there was less pain, because Curious Incident had taken off.”

More glory is to come. Even before publication the film rights had been sold in a joint deal to Warner Brothers/Heyday Films/Brad Pitt and Brad Grey. Steve Kloves, who adapted Harry Potter and Wonder Boys, is to write and direct. "He's also a dead dog specialist," says Haddon, pointing out that in Wonder Boys Tobey Maguire shoots a pit bull terrier and drives around with it in the boot of a car.

Having resisted the lure of the two-book deal, Haddon is working at his own pace on a novel about "a recently retired man who experiences a florid nervous breakdown, but with funny bits". He is pleased to discover that he would rather settle down to work than rush out to blow his new-found wealth on a sports car. "This whole thing has been bizarre: writing what I think is quite a literary novel and then getting sucked into the jaws of mammon." He beams.