When Mark Gatiss first moved to London he stayed on fellow League of Gentlemen star Steve Pemberton’s floor in Liverpool Road.

The 46-year-old from Sedgefield in Durham now lives “around the corner” in a Georgian house with his partner Ian and dog Bunsen.

“This whole north and south London thing, I think there’s something psychological about us northerners settling in the north of the city,” he says.

A huge fan of Victoriana, the actor and scriptwriter once recreated a Jekyll and Hyde-style laboratory in the house.

“I filled it with old stuff, painted the walls blood red and put in gas lamps. I built a folly but it was a valuable lesson that what you want at eight you don’t want at 40 because I realised I never did anything in it.”

Mark’s obsession with 19th century crime fiction has stood him in good stead when co-creating the hugely successful updating of the Sherlock Holmes stories with Steven Moffatt.

He grew up reading Conan Doyle’s originals and says he and Moffat were “very confident” they had made something good.

“Good doesn’t matter, a lot of good things never get noticed,” he notes wryly. “But I am thrilled and humbled by its success. Benedict [Cumberbatch] became a star literally overnight. That’s not supposed to happen but it did and we’ve only made six episodes.” But Mark is all too aware you can’t write for success. “As far as possible I have always made things I’d like to watch myself.

“When something becomes successful it can get a vocal fanbase who feel they own it and can dictate terms, but that’s suffocating and if you ever listen you are sunk. You have to do it for yourself.”

Dr Who, was one such project of personal passion. A childhood fan he has to remind himself he’s writing “a new show with a new audience discovering it for the first time”.

“You have to make sure you are not making if for your eight-year-old self, you cannot be nostalgic for a past version of it.”

He loves it so much that he’s just written a 50th anniversary “biopic” about the show’s creation, titled An Adventure in Space and Time.

Mark, who spent the spring writing new episodes of Sherlock as well as two for the Dr Who series and a Poirot, is taking time out to play King Charles I. In Howard Brenton’s 55 Days. The man who as a League of Gentlemen member donned an array of headgear to transform himself into residents of the fictional town of Royston Vasey, will sport a Civil War-era cascade of curls as the ill-fated monarch.

“I have always wanted to play Charles I and been fascinated by him and the period although I am firmly on Cromwell’s side.

“Although I get to wear ‘the wig’, it’s not quite how I expected. I’m not a king with a court and all that deference – the 55 days is the period between his imprisonment and execution.”

The League of Gentlemen formed at drama school Bretton Hall and Mark’s writing began when he and Steve Pemberton took two plays to the student drama festival. As comedy geeks, the league learned their craft from their intimate knowledge of the 70s classics they watched as children, from Dad’s Army to The Good Life.

He ponders the possibility of a 20 year reunion in 2019, but admits the controller of BBC2 is hardly hammering on the door for a new script.

55 Days runs at Hampstead Theatre until November 24.