COLIN SALMON was once the luckiest man alive. As Detective Sergeant Bob Oswald in Prime Suspect II, he fulfilled the hopes and dreams of a generation of hot-blooded British males by, well, having Helen Mirren. In bed. Naked as the day she was born.

It was all an act, of course. But a good one. So much so that there was much talk subsequently of taking his black cop hero into a spin-off film. Several years later, it has finally happened. In Deep Secrets, a taut and violent thriller made (in a rare moment of cross-channel co-operation) by Granada Television for the BBC's Screen One series, Salmon plays Nolan (no first names, no pack drill), a Detective Sergeant not a million miles removed from the Prime Suspect character.

Nolan is a fast-track desk cop who gets offered the chance of going undercover to investigate a savage murder among low-life drug dealers in Manchester. The film co-stars Amanda Donohoe, as the estranged wife of a ruthless Turkish gangster, Ann Mitchell as a tough CID boss, and newcomer Sophie Okonedo as a detective living deep cover among the bad guys.

Salmon, most recently seen in villainous mode opposite Amanda Burton in Silent Witness, did much of his own research for his latest role.

``I have a police contact in London who's worked undercover. I talked about it with him but it was difficult to get much out of him. These guys won't talk about past cases. But what I did realise was that there are some real similarities between acting and going undercover.

``It's all about psychological game-playing, role-playing - and that's acting. Some of the cops who do it enjoy it. Others find it very difficult. But, unlike actors, it's not about the bad reviews they get. If they give a bad performance then it's potentially a kneecapping or worse for them,'' he explains, somewhat chillingly.

Deep Secrets opens with Salmon's character, Nolan, being sent in to try to salvage a lengthy undercover operation which has gone pear-shaped after the death of an informer. Nolan insinuates himself into the local underclass, posing as the long-lost husband of Honey (Okonedo).

Salmon adds: ``When undercover cops are thrown in to these situations they don't have a real life for months on end. They can't go back to their friends or their families. They just have to keep playing this dangerous role. And it really is dangerous for their lives depend on it.'''

It wasn't so very long ago that Salmon, 34, was keeping the wolf from the familial door by busking in the streets of Kilburn. At one point he seemed destined for the football field - he even played for England Schoolboys. But instead he tried for a career in music (he's been playing the trumpet since the age of seven).

While busking outside the Tricycle Theatre in North London a couple of actors invited him inside to watch the show. He became a familiar figure in the audience and, eventually, the actors wrote a small part for him. From there he landed the Prime Suspect role. Between then and now he has steadily built up a career with parts in Lovejoy, Soldier Soldier, and Shine On Harvey Moon.

Today Salmon, who's married with two young daughters (``little angels'' he calls them), combines his acting career with the occasional return to professional musicianship. In the past he has played at Ronnie Scott's club in Soho and toured with Ben E. King, Eddie Floyd, and Curiosity Killed The Cat.

``I don't think there's any conflict between my acting and playing music. Being a sideman is okay but I'd really want to do my own music,'' he explains.

Though it's billed as a one-off film, Deep Secrets has all the makings of a future series. The actor, who is lined up to make two feature films over the next year, would be well pleased if that were the case. It sure beats blowing a trumpet outside the Tricycle Theatre.n.Deep Secrets can be seen on BBC1 on Saturday, April 6.

Going in deep: Colin Salmon.