The modern Renaissance man talks about his new role as Charles I

There are many things to ask Mark Gatiss: Did Benedict Cumberbatch’s Sherlock die in the last episode? How do you write dialogue for Daleks? and What is ‘the special stuff’?

But what I end up asking about is wigs. As a founder member of macabre comedy troupe the League of Gentlemen, he donned an array of headgear to transform himself into residents of the fictional town of Royston Vasey.

A mullet for special needs jobseeker Mickey, hippy tresses for naturist aunty Val, and mutton chop sideburns for sinister butcher Hilary Briss, whose ‘special’ meat gave everyone nosebleeds.

Elsewhere he’s proved versatile in roles as varied as Bamber Gascoigne in Starter for 10 (if there’s ever a biopic he’s a shoo-in) and suave manipulator Mycroft Holmes in the stupendously successful Sherlock co-created with Steven Moffat.

He’s soon to sport the full enchilada, a Civil War-era cascade of curls as he plays ill-fated monarch Charles I in Howard Brenton’s 55 Days.

“As Mel Brooks says, ‘it’s good to be the king,’” he beams, relaxing on a sofa between rehearsals at Hampstead Theatre.

“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.”

For all the grotesque no-hopers he’s channelled, Gatiss in person is tall, slim and dapper. Turned out in casual jumper and brogues, you sense a keen intelligence and an affability tempered by the wariness of those who’ve become habituated to answering questions from journalists.

The Sedgefield-born 46-year-old, who lives in a Georgian house in Islington with his dog Bunsen and civil partner, is a modest renaissance man, whether writing scripts for TV shows Dr Who and Poirot, presenting and writing documentaries on pet subjects like the Hammer Horror movies, or doing an Alan Ayckbourn at The National Theatre.

He’s such a fan of Victoriana, he once created a Dr Jekyll-style lab at home, complete with blood-red walls and gas lamps, but ditched the idea when the room became a relic that never got used.

Resolutely modern, he’s latterly embraced twitter and word processing, and Sherlock fans will be delighted that he’s written new episodes, as well as two more for Matt Smith’s Dr Who - plus a 50th anniversary “biopic” about the show’s creation, titled An Adventure in Space and Time.

Often asked what he would choose between performing and acting, he protests “unless someone holds a gun to my head I’m not giving anything up!”

“I have always done both and when it works best it’s a lovely balance. This year I did a play at the Donmar in January, spent April onwards writing and now I am doing this. If I had just been shut up at home writing I might have gone a bit mad, but doing this play is refreshing, I am out of the house with other people, being challenged and having a good time. I come back to writing from rehearsal energised.”

But he admits to the pressure of creating, writing and acting in a prime time show.

“Sherlock is a big responsibility an awful lot of work goes into it and it can be nice sometimes to do a guest slot and not be in control.”

Gatiss grew up reading Conan Doyle’s originals and he and Moffat were “very confident” they had made something good in updating the stories, whose success he describes as “thrilling and humbling”.

“Good doesn’t matter, a lot of good things never get noticed,” he notes wryly. “Benedict became a star literally overnight. That’s not supposed to happen but it did and we’ve only made six episodes.”

Gatiss 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 the sheer imaginative freedom of a show that allows the central character to regenerate every few years and to visit anywhere in time and space.

“For a few marvellous months (original producer) David Whittaker opened scripts that started ‘It’s the year 5 billion and the world is on fire!’

“It’s the most amazing concept. Full of the most brilliant, original ideas that were almost entirely borne out of serendipity or budget constraints. Can’t afford a spaceship? Perhaps it can look like something ordinary.”

The League of Gentlemen formed at drama school Bretton Hall and Gatiss’ writing began when he and Steve Pemberton took two plays to the student drama festival.

“It was a very exciting time. We were young with a sense that anything was possible. We were frustrated by not getting much support from our drama school but that made us very self reliant.”

While he and Jeremy Dyson channelled “a sort of domestic horror of embarrassment and failure”, Pemberton and Reese Shearsmith had a “gothic sensibility” evident in out there characters like Papa Lazarou and Edward and Tubbs.

“Dare I say it I was more Nuts in May and Alan Bennett but hugely influenced by Victoria Wood and sit-coms like Rising Damp which was a very painful kind of comedy.”

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.

“Today there’s a reluctance to commit to the fact that all great comedy has a tragic centre, it’s about people being trapped, and it would be lovely to see one with people in it who were not attractive but pathetic.”

By series three, the league decided to call it a day because: “doing something for the sake of it is death.”

“We were together for almost 12 years and like any collaboration you reach a point where you want to do different things, thankfully we have all been successful. But like Abba, we never actually split up”.

He teasingly ponders the possibility of a 20 year reunion in 2019 when they might revisit the denizens of Royston Vasey, but admits the controller of BBC2 is hardly hammering on the door for a new script.

As for Charles, Gatiss is well stuck into what he calls a “rather neglected” moment in British history.

“It’s been reduced to Cavaliers; fun, roundheads; dull, but it’s the beginning of modern democracy.

“Charles was no different to any king that preceded him except he was suddenly out of step with the times. He had a dangerously fanatical edge and was a zealot about the divine right to rule, with no sense of compromise that a wiser monarch might have.”

Brenton focuses on the political revolution that involved forging a new country without a king and says Gatiss, has “rather brilliantly made it into a political thriller”.

“Cromwell has this speech that ‘we are not just trying a tyrant, we are inventing a country’. He’s the man of the hour, the only person who can push it through by controversial means. He’s an arch politician, a genius, a soldier, even though the experiment in republicanism didn’t work, Britain was never the same again.”

55 Days runs at Hampstead Theatre until November 24.