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From the outset, Five Daughters relied heavily on the co-operation it received from Suffolk Police. And that help was never more appreciated than when Ian Hart began work researching the role of the head of the investigation into the Ipswich killings, Detective Chief Superintendent Stewart Gull.

"I went to see Stewart Gull at his office in Ipswich. He spared me some time even though he is a very busy man. I wanted to try to get an idea of who he was, I wanted to find out the little things: did he like football, was he a family man, that kind of thing. I wasn't going to do a slavish impersonation of him, but I wanted to understand him better," he explains.

What Hart discovered was unglamorous yet quietly inspiring. "It turned out he is a career policeman. His dad was a copper and he joined the force via the police cadets when he was young. He is what his title says he is," he explains.

As a result, Gull is far removed from the kind of detective we are used to seeing on our televisions screens today.

"The conventional television policeman these days is flawed; he is sleeping with lots of women or has a drinking habit. We have to see the strain and the weight of it all, we have to see how it's taking its toll. But he is the antithesis of that. He is a career policeman with a wife and a family, doing his job," he explains.

By spending time with Gull, Hart also began to understand that when the bodies of five young women began being discovered around Ipswich in December 2006, the career policeman found himself leading one of the most extraordinary investigations in British police history.

"I talked to him about the investigation. No police force in Britain had had something of that nature happen on their doorstep before. It was a very quiet town. Even the Met don't get cases like that. They get gun crime and individual murders but they aren't connected," he says.

"As much as they were prepared for a case like this through their training, this was something completely different."

Hart grew to appreciate how Gull faced enormous strains during the investigation.

"There was a lot of pressure on him not just from his superiors within the police force but from the press who wanted information all the time," he says.

Ultimately, Gull's professionalism and persistence paid off.

Meticulous investigation led to the discovery of DNA evidence at the scenes of three of the women's murders which in turn led Gull and his team to investigate a 48-year-old forklift truck driver, Steve Wright. He was arrested in December 2006 and tried in February 2008 when he was found guilty of committing all five murders. He is now serving a whole life sentence for the crimes.

Like the rest of the cast, Hart found himself deeply affected by the drama, in particular, during the scenes in which Gull and his colleagues discovered the dead bodies of two of the women in woods.

"They were the scenes that had the biggest emotional impact," says Hart. "You can put yourself in that place very easily. That image brought it all home to me to a degree. It wasn't just a photograph on a wall in an incident room. It wasn't removed from reality."

At that moment, more than any other, he understood the pressure that Gull and his colleagues must have been under during the six-week-long investigation.

"I felt what it must be like to find that and then go back to the office thinking 'I've got to solve that crime'. Especially as I believe he has a daughter himself. There must have been a tremendous emotional response to that."