FOR 11 years Graham Darbyshire thought he'd got away with one of Lancashire's most brutal rapes.

And he would probably still be a free man today - had he not left his DNA at the scene.

At the time of the attack in 1995, the standard of DNA technology meant that there was a 1 in 10,000 chance that a suspect's sample would match that of an innocent man.

That meant DNA could help investigations but could never be conclusive.

In a town like Blackburn, for example, there could be nine other people with a matching DNA profile, given the one-in-10,000 probability.

But over the past decade there have been incredible advancements in forensic technology.

Now that 1 in 10,000 chance can be narrowed down to a one-in-a-billion chance.

So if police get a DNA match, guilty pleas, as they did with Darbyshire, typically follow.

The trail to Darbyshire began to unravel in the summer of 2005 when detectives on Greater Manchester's cold case unit launched a re-investigation into a vicious indecent assault in Salford in July 1993.

Evidence was re-submitted to the DNA laboratory and - thanks to advancements in forensic science - a DNA sample that had not previously been spotted was discovered.

The profile was checked against the national database and gave a match to the 1995 Billinge Wood rape so officers knew both attacks were committed by the same man.

Officers could not match the DNA with anyone on their databases and so searched to see if they had the DNA of any relatives of the attacker.

However the search is not an exact science and it threw up 38 names.

But one of them was a relative of Darbyshire, who it is understood had a conviction for shoplifting.

And right away Darbyshire was identified as a prime suspect from the 38 because he had a conviction for an eerily similar attack to the Billinge Wood rape.

In 1984, he had raped a woman walking her dog in Leyland and served a prison sentence for the attack.

As officers looked into Darbyshire's record they noticed he had lived in an area of Wigan near to Boothstown, where the Salford attack had taken place.

The anticipation that they had finally got their man was building for DI Mark Rothwell and DC Kate Kennedy, who were working on the re-investigation.

DI Rothwell said: "At this point we made the decision to arrest him. We didn't want to swab him and give him the chance of disappearing or, as can sometimes be the issue in these cases, self-harming.

"We were very hopeful but the inquiry team had had a lot of leads and I am pretty sure they thought they had got the right man several times until the swabs came back negative. It had been a real rollercoaster. But when he came on the radar we were hopeful."

Detectives traced Darbyshire to a house in Leyland and he was arrested on May 22 this year. DI Rothwell added: "Darbyshire did not say a word. There was no protestation of innocence. He was impassive. For eleven-and-a-half years he must have thought he had got away with it."

Police pulled out all the stops to make sure they quickly found out if Darbyshire was indeed the man.

Normally, they would take a DNA swab which would be sent away for analysis. The arrested man would be released on bail while officers awaited the results.

But police had the swab taken by motorcycle to the London laboratory so the result could be obtained while Darbyshire was still in custody. And it came back as a match.

But before this news DI Rothwell knew he had got his man.

He said: "During the questioning, I knew straight away when I looked at him and he was biting his nails exactly as the victim had described. It was the last piece of the jigsaw for me."

Darbyshire had been denying the attacks. But when detectives put the DNA match to him he changed his stance to no comment.

Inspector Joe Kellet, who worked extensively on the original investigation, was brought back by the new inquiry team to arrest Darbyshire.

And the gesture provided the officer with a happy conclusion to an 11-year-old hunt that left a mark on him personally.

Insp Kellet, who now works for the force's training school, said: "I felt very privileged to make the arrest and it was very satisfying, not just for me but for everyone who had worked on the case.

"But most importanly we were delighted for the victim as it must have been a tremendous ordeal for her and it brought closure."