Out of the mist of the Wicklow mountains, The League of Gentlemen's demon butcher squelches through the bog, plopping out his false teeth as he walks. The locals round here, in "the garden of Ireland", would sooner fix you a sandwich than burn you at the stake for stepping into the village shop, which is how the grotesque characters of the comedy show routinely threaten outsiders. But County Wicklow is currently doubling as a post-apocalyptic Royston Vasey, the nightmarish northern village in which the cult series is set, for the League of Gentlemen film. And so Wicklow's windswept landscape is now home to the cult TV characters once described as Twin Peaks meets Mervyn Peake meets Peak Practice.
Of course, Wicklow has strange phenomena all its own. In the back room of a post office in a nearby village, a statue of the Virgin Mary famously wept blood for all to see. The local extras for one scene were so well chosen, says Mark Gatiss, co-writer and co-star of the TV series and film, that they reminded him of his childhood growing up opposite a Victorian psychiatric hospital in County Durham.
Gatiss, 38, is sitting by the radiator in a trailer, smoothing the bloody apron worn by Hilary Briss, the village butcher he plays in the hit series. It is 10 years since he and three friends from a Leeds drama college - Steve Pemberton, Jeremy Dyson and Reece Shearsmith - first performed the the League of Gentlemen sketch show, at the Cockpit theatre in London. A Perrier award at the Edinburgh festival led to three TV series and a stage tour, and now a £4.2m feature film.
The success of the TV series has allowed Gatiss to indulge in some of his personal obsessions. Having already written four Doctor Who novels (the first published while he was penniless and sleeping on Pemberton's sofa), he has written an episode of the new Doctor Who series. But his latest venture is perhaps his most unlikely. An offer in 2002 by the publishing house Simon & Schuster to publish "whatever he wrote" has resulted in what Gatiss describes as his first "grown-up" novel.
The Vesuvius Club, written in 18 months alongside the League of Gentlemen screenplay, is a pastiche Edwardian James Bond thriller about Lucifer Box, the most fashionable portraitist of his generation and a government assassin. The Royal Academy of Arts is revealed to be the headquarters of a bizarre arm of His Majesty's secret service, run by a dwarf "M" character called Joshua Reynolds. "I thought, what if the Royal Academy was so much more than it appeared, what if it was a front?" says Gatiss. Of course, Anthony Blunt, the British agent and Soviet spy, was an art historian and Surveyor of the Queen's Pictures. And the painter Walter Sickert, who is referred to in the novel, was Jack the Ripper, according to US crime writer Patricia Cornwell. So is he simply developing an existing idea? Gatiss winces. "I can't bear the Patricia Cornwell thing. It's my bete noire. First, because it's the oldest theory in the world. Second, Sickert was in France for the whole time the Whitechapel murders were happening. So unless he had very long arms, it wasn't him."
The most interesting thing about the book is that, aside from the old newspaper adverts lining its cover ("We buy old or disused false teeth"), the beau monde of Lucifer Box could not be further from the suffocation of Royston Vasey. In fact, it seems clear that Gatiss dreams of being Box himself. He has installed a Victorian laboratory in a room in his north London house, fully equipped with gas burners and a stuffed cat. It was the realisation of a childhood dream, he says, except the novelty wore off when he realised he never used it: he was just showing it to dinner-party guests and shutting the door.
Gatiss's embrace of escapism began when he was a child growing up in a village in County Durham. "I went back to Heighington once, and discovered it was the most idyllic English village with a maypole and water pump. I thought, why didn't I realise this before?" he says. "But Newton Aycliffe, where I went to comprehensive, was a postwar town and I hated it. I used to wish I had been brought up in Oxford or somewhere pretty. I retreated into Sherlock Holmes. I wanted to live like an 1895 detective, not in a grim post-industrial town."
Both his parents worked in the psychiatric hospital opposite: its patients he viewed simultaneously as completely normal, and endlessly fascinating. "I used to go and watch films shown there. I remember watching Chitty Chitty Bang Bang and being almost as frightened of the people sitting around me as [of] the Child Catcher. The faces and personalities were true northern Gothic.
"In my first year at college I got a job there as a gardener. I got a totally different view of it. One patient, Peter, would be walking back and forth every day. He would come and talk to me, he was so sweet. And he had been put in there when he was about 15. Three of his brothers and sisters had gone into other institutions. He would say to me: 'I'd just like to see my sister.' " One day, while he was clearing away some grass, he found an old plaque which read: "Aycliffe colony for the mentally defective."
After drama school, he went to London to "scratch a living" as an actor. When work slowed, he sat up writing Doctor Who novels. When the first copy of his first book arrived, he took it to the launderette: "I sat there staring at it because it had my name on it, willing more to come out of it."
Gatiss wrote The Vesuvius Club, he says, because he could never find the book he wanted at the airport. "I have memories as a child of going to visit lots of elderly relatives in an Alan Bennett-ish way. They always had support bandages and what Bennett calls the three Ds: dirt, death and disease. Conversation was all about people dying off: 'Have you heard, so-and-so died?' It used to really depress me. That's what I wanted to escape from."
Is the novel perhaps less of a risk than translating cult TV comedy to the big screen? He takes a slow breath. "Yes, there is always a risk, and I'm aware of the unhappy history of it. But there are happy examples too. Monty Python is now more recognised by the films than by the TV series.We have tried hard to make sure it's accessible, and not just a cheap rip-off."
Then he disappears back to the set and into the sheeting rain - made worse by the sadistic addition of a rain and wind machine - explaining: "I've just got to stick a knife in someone's throat."
· The Vesuvius Club is published by Simon & Schuster on November 9, price £15.