Get your first look at Sherlock, Season 4, starring Benedict Cumberbatch and Martin Freeman

Sherlock mania – second only in hysteria and volume to Beatle mania – is gripping the streets of north London.

The TV Guide is corralled behind a crowd barrier in the vicinity of Euston Station. We are standing alongside thousands of fans who have journeyed from as far afield as China and Japan. They have been lining up since before dawn along North Gower Street which, as all card-carrying Sherlockians know, doubles for Baker Street in the BBC's global hit drama.

There is uproar when the lead actor Benedict Cumberbatch, in Sherlock's trademark Bellstaff coat, magically appears out of a side street to start filming in front of the "consulting detective's" famous front door. The cheer from the assembled masses can no doubt be heard south of the river. The game is on.

Benedict Cumberbatch plays the famous fictional detective in Sherlock.

Chatting to the TV Guide in a nearby hotel before filming starts, the cast and crew are equally delighted that the show is returning with a full series after a two-and-a-half-year break, a gap partially enforced by the fact that its two principal actors, Cumberbatch and Martin Freeman, have become megastars on the back of Sherlock.

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Forty-year-old Cumberbatch confesses that he is having the time of his life on this fourth season.

Martin Freeman plays John Watson in Sherlock.

"I love it more than ever. I also love the character more now because I've got to experiment with playing different sides of him."

The actor says that the fourth season has been demanding, and yet richly enjoyable to make because Sherlock is, "Troubled and challenged, at the top of his game and at the very bottom. So there are huge highs and huge loads, and you really find out who he is."

Sherlock has long since passed from programme to phenomenon. Now broadcast in 240 territories across the world, the series has won 12 Baftas, seven Emmys and a prestigious Peabody Award.

Sherlock creator Mark Gattis (left) as Sherlock's brother Mycroft.

The global success of the show has taken its producers aback. Mark Gatiss, 50, the co-creator with Steven Moffat, remains astounded that the show has become such a sensation all over the world.

Gatiss, who also stars as Sherlock's devious older brother Mycroft, observes that, "Sir Arthur Conan Doyle was baffled by the success of his creation towards the end of his life.

"We are not comparing ourselves with Conan Doyle, but we're not baffled – we're just grateful.

Supplied Benedict Cumberbatch says the Sherlock character 'still maintains a fascination'.

"It's like bottled lightning. Something happened the moment the writing and the casting came together. It's just one of those shows. It will never happen again."

Moffat and Gattis are much less keen to talk about how the new series develops.

"We could give you three words describing it that actually have nothing to do with a series," Gatiss teases.

"Wolverhampton. Peanut. Throttle," Moffat adds, with a grin.

What we are permitted to know about the fourth series, entitled The Six Thatchers, is that Sherlock is unsure about the continuing involvement of his apparently dead foe, Moriarty (Andrew Scott.)

All we know for certain is that Sherlock will be confronted by another seriously demonic and devious baddie, Culverton Smith (Toby Jones), who is the tropical diseases expert and poisoner in the Conan Doyle story, The Adventure Of The Dying Detective.

So what of the future? Can we look forward to another series?

Cumberbatch is hopeful.

"I'm proud of what we've done. But yes, of course, it still maintains a fascination, otherwise I wouldn't come back for more.

"I've got a lot going on in my life at the moment, thankfully. I'm very lucky as an actor. So it's not just about employment. It is about wanting to do it and I'm very lucky to be in that position. And also to have a character like this who I still really want to play."

Freeman agrees. "My feeling is always that if you enjoy something, then let it breathe and give it a chance, but stop something when you're no longer enjoying it."

After having a ball on the last Victorian special, Freeman can also see the potential for another series set in a different period.

So what other era would the actor like to see the characters transported to?

"I'd like to do 1066," he says. "It would be about how Sherlock and John handled the Norman Invasion."

Sherlock, TNVZ 1, returns Sunday January 29.

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