It is not often that someone gets the chance to overturn not one but two national institutions, but the bestselling Irish author Eoin Colfer has managed both. On January 5, a 13-part adaptation of his detective series Half Moon Investigations will replace Grange Hill on BBC One and later this year he will publish a sequel to Douglas Adams's celebrated cult sci-fi comedy series The Hitch-Hiker's Guide to the Galaxy. No wonder he sounds exhausted when we speak.

“It's been a very strange time,” he admits. “I feel unsettled as I haven't since the first Artemis Fowl book was published.” That first book, about a boy genius who works out that fairies really do exisit and kidnaps one to restore his family fortunes, was famously described as “Die Hard with fairies”. Brilliantly funny, suspenseful and inventive, it propelled the Wexford English teacher into the super-league of new children's authors, enabling him to give up his day job and produce at least a book a year since 2000. Since then, a series of novels have continued his success, ranging from The Legend of Captain Crow's Teeth (for 6+) to the utterly brilliant Jules Verne-like suspense novel Airman (11+). Yet although the Artemis Fowl series remains one of children's most longed-for Hollywood adaptations, nothing of Colfer's had actually been made - until the BBC came along.

“I agreed with weariness, thanking God for the money but with no expectation of anything being made,” Col-fer says. “Every novel I've written has been optioned for film and I've even had Airman optioned by Robert Zemeckis (who made Beowulf and Back to the Future) only they haven't yet happened. But the BBC was so keen they were casting it and writing the scripts even before the contract was signed.” It's easy to see why. Colfer's idea of a schoolboy who sets himself up as a kind of junior Sam Spade after taking an internet course in detection, and gets involved with school crime and a “family of Barbies”, is irresistible. One of the funniest writers alive, Colfer's deadpan Irish wit has been transposed to Glasgow, but still works just as well.

“They've kept the one-liners and the repartees, and I'm very happy with it. My kids are just of the age where TV is a big part of their lives,” he says. “Three of the episodes are from the book, but the rest have been written by someone else and they've got a good Celtic sense of humour.” Stepping into Douglas Adams's shoes has, however, incurred the wrath of many fans. Having turned down requests to write sequels for other famous books, he agreed because he loved the books as a schoolboy and thought that it would be fun.

“I think it's going to be a good book, not a Douglas Adams book, but one that will stand on its own,” he says, being two thirds of the way through it. “Douglas leaned towards that Monty Python humour, which I think I share - the sense of someone normal being dropped into a ridiculous situation.” Yet Colfer's best-known work is all about the reverse - about someone very abnormal, like a child-genius or someone who can (as in The Supernaturalist) see the dead. He agrees, and says that he has in fact introduced one or two characters of huge intelligence in And Another Thing, but what he's most enjoyed is bringing back really stupid characters such as Zaphod, with his two heads, four arms and reputation as the “worst-dressed sentient being in the known universe”. It's a very male sense of humour and Colfer, who is the second in a family of five boys, with two sons of his own, is completely attuned to the kind of thing that Adams fans love. His reverence for the existing five books is genuine.

“The Guide is a slice of satirical genius. A marvel of quantum tomfoolery. A dissection of the absurdities of our human condition. My first reaction was outrage that anyone should be allowed to tamper with this incredible series, but then I thought, why not? I'm always looking for a challenge. It's got nothing to do with money - I get more for doing another Artemis book - but I thought I could take the risk.”

The vitriolic reaction of some fans, whom he describes as “very vocal and organised” has taken him aback, even if what he writes can't affect the original books and is more like the ultimate act of fandom. It does seem extraordinary that someone bursting with an unstoppable flow of his own creations, with a wit, charm and storytelling genius many would envy should have chosen to expand somebody else's fictional universe - but if anyone can do it, it's Eoin Colfer.

Half Moon Investigations begins on BBC One on Monday at 4.35pm. The book is published by Puffin

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