The gig isn’t exactly secret, but it’s small and obscure. Magic Alex are performing late at a big, dingy Victorian pub in North London. Inside, though, the atmosphere is darkly cosy. When the band arrive they are surrounded by friends and admirers, all kisses, big hugs and claps on shoulders. They have been together for a decade, but this is their first gig in two years. I feel as if I’m crashing a private party. In the middle of it all is John Simm, the band’s bassist, but better known as one of our most impressive young screen actors, and he is clearly loving every minute.

They begin with a belting version of Velvet Underground’s I’m Waiting for My Man and Simm is away, lost in the energy of his guitar playing. It’s as if he is a Mancunian drama student all over again, a fanatical devotee of Oasis and the Manchester club scene. Yet many television viewers will know him only for his recent work in quality dramas. In 2002 he was an unnervingly psychotic Raskolnikov in Tony Marchant’s brooding adaptation of Crime and Punishment, and last year he played Cal McCaffrey, the conflicted investigative journalist in the critically acclaimed State of Play. His latest big drama, Sex Traffic, which starts on Channel 4 on Thursday, will only enhance his reputation as a serious player.

This time he plays Daniel Appleton, a self-effacing translator working for a refugee charity who stumbles on the horrible realities behind the kidnapping of “Natashas”, Eastern European girls who are forced into prostitution in the West, and tries to help two sisters from Moldova.

Yet he began his career playing much less altruistic characters. He starred in The Lakes (1997), Jimmy McGovern’s raw drama about the aspects of Lake District life that you won’t find in Wordsworth or Beatrix Potter. Then came Human Traffic, a cult film about the club scene, and the Manchester club film, Twenty Four Hour Party People. So Simm was a star of a drugs and violence subculture long before his growing mainstream fame.

“If I get recognised in public, it’s mainly for Human Traffic,” he says. “It’s a real underground clubbers’ film, even in America. I had to stop going to clubs because everybody’s off their face, and they’re coming up to me and going, ‘Oooh’.”

Simm has a three-year-old son, Ryan, and he is glowingly aware of his family responsibilities these days. “I’m much more careful now. I don’t want to die!”

In his early years Simm had a reputation for being spiky and taciturn with interviewers, which stemmed from a rather hedonistic period. “It lasted throughout most of the 1990s, actually,” he admits. “Magic Alex used to do tours with Echo and the Bunnymen, and if you hang around with musicians . . . I had a great time. So when I did Human Traffic I knew what I was talking about. At the audition they asked, ‘Have you ever taken ecstasy?’ and if someone said ‘No’, well, forget it.” It was in what he calls “a mad year” that Emma Bunton, aka Baby Spice, became one of many flings that attracted the attention of the tabloids. But he doesn’t feel that his youth was misspent: “I don’t think it ’s a big deal — ‘John Simm took drugs!’ Wow! It’s not a big story.”

The experience also put him off the idea of celebrity. “I try not to get overexposed and appear on things such as Celebrity Mastermind. It was De Niro or Gary Oldman or someone who said if an actor gives too much of himself, and everybody knows everything about him, it’s difficult for people to believe he’s in character. It goes against what you’re trying to do.”

Nevertheless, when we meet in a bare conference room at a publicist’s in Bloomsbury, he is relaxed, friendly and talkative, casually dressed (he will smarten up for THE EYE’s photographer) in jeans, trainers and a Velvet Underground T-shirt. He seems a slight figure; short and slim. His distinctive screen presence comes from his boyish face — he looks 34 going on 14 — combined with a certain menace, an air of lurking pugnacity, like a cherub who would punch your lights out if you crossed him. But he also has a restless mercurial quality, and exudes a sense of scepticism and moral intelligence.

He has arrived fashionably late. He had been working long hours filming the screen version of the stage hit Blue Orange, he explains, and had trouble dragging himself away from precious time with Ryan. Home is in the mildly bohemian suburb of Crouch End in North London. He used to live in neighbouring Muswell Hill. Both areas are crawling with actors. “Muswell Hill reminded me of Trumpton,” he says. It’s all a far cry from Nelson, the small mill-town near Burnley where he grew up.

His father was a northern club entertainer. They lived on a Nelson estate and Simm went to the local comprehensive. I mention that my mother came from Stalybridge, another mill town satellite of Manchester. I have a nostalgic affection for those Lowry townscapes, but weren’t they bleak places to live after the cotton industry died?

“Yeah,” he says. “Awful! I think they’re still trying to pull themselves out of it now. But also there’s this BNP thing in recent years. You know, Burnley and all that, which is very, very embarrassing. It’s just ignorance, absolute pig-ignorance. The parents are passing it on to the kids, so nothing changes. When you live there you don’t question why it’s like that. When I moved away I saw it for what it was.”

He didn’t linger. “Nelson is one of those places you want to get out of quickly if you have any ambition. I’d gone by 16.” A teacher had pointed him towards drama after he had mentioned an enthusiasm for James Dean in Rebel Without a Cause. “It was the very first thing that I remember watching and thinking, ‘Wow! That looks like a very good job!