Fred and Rose West detective recalls what it took to solve the case – and get confession

Real-life footage and eerie voice recordings of Fred and Rose West are also featured in the documentary, along with interviews from the people who worked tirelessly to solve the case and bring them to justice.

"I'd been on a rest day and I think it was the Saturday or the Sunday [when] they called me to come in on the Monday, so that was the first indication I was going to be involved," detective constable Russ Williams exclusively told RadioTimes.com about how he came to work on the Fred and Rose West case.

"It's really important to stress that up until this point, it was very low level. There was a side of the Gloucester division who was looking or trying to trace Heather. The rest of the police force weren't aware. It was a missing person. It was a person who the police could not find, but that was entirely the length and breadth of the investigation trying to find Heather."

What transpired was nothing the police could have imagined – as the other West children told people that Heather was underneath the patio and if they did anything wrong, they too "would end up like Heather", Williams explained.

He added: "If you can imagine that sort of disclosure coming from young children, then it's really quite a stretch of the imagination, because I'm sure there's parents out there that at that time would make similar threats of punishment if their child did anything wrong."

So where do the police go from here? Well, in a world without mobile phones and being able to track messages and locations, the police looked to see if Heather had been to the doctors or the dentist amongst other things, "and all of that was coming up blank".

In addition to Heather's disappearance, there had also been concerns about abuse the West children had been suffering, which is where the investigation kicked off according to Williams, and the police were granted a warrant to access their home.

"We found human remains."

But what was "interesting" at the time, was that Fred West had already begun to make early admissions of being responsible for Heather's death, and where he told the police her remains would be found, was actually where other remains were uncovered.

Williams continued: "The option was to completely excavate the garden and that then led to the discovery of human remains. And as this was going on, Fred was becoming aware. He was making more and more admissions, to the point where he disclosed this, 'I admit to approximately nine further killings.'"

It was with this knowledge, that the police "started taking the house apart" and soon, "that's when the real horror of what was happening at Cromwell Street began to dawn on people".

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For Williams, he believes that Fred began to "relish his opinion" and enjoyed that he was becoming "infamous" across the media, but towards the end of his interviews "the psychological pressure" began to weigh on him.

Williams explained: "Although Fred had an appropriate adult, Janet Leach, who is there to look after his interests and facilitate the communication between them – to make sure cognitively he was aware of what was happening – his mental health did start to suffer, and it was at that point that the interviews began to stop, and the reliability and the accuracy of the information coming from Fred started to need to be questioned.

"Because even if he went into a police station now and made a full and frank admission, the police would still be required to find cooperative evidence to support that admission, because there are some people out there who will make admissions to things when they hadn't actually committed those offences."

While being questioned by the police, Fred West confessed to at least 12 murders, the earliest being Anne McFall, a nanny he had employed in 1967 when he was married to Catherine 'Rena' Costello to care for their child Charmaine – both of whom he murdered.

The remains of nine women were found buried at 25 Cromwell Street, including Heather and Charmaine West as well as Alison Chambers, Thérèse Siegenthaler, Shirley Hubbard, Lucy Partington, Juanita Mott, Lynda Gough and Carole Ann Cooper.

"Fred and Rose were comfortable in disposing of the bodies where they lived," Russ told RadioTimes.com. "The bottom line is, that is what we were working with, that was their profile.

"That's where they were disposing of the body. That's where they would bring the children back to. That's where they would imprison them, tie them up, torture them, use them, eventually kill them and dismember and bury them. It was all within the areas where they lived."

Initially it was just Fred who was being looked into for the crimes, but as described in the documentary, the more they began to dig into Rose, the more became apparent.

Williams explained: "When you look into their backgrounds, you can see how their whole demeanour, their own psychology was affected by their upbringing. We had these two damaged individuals coming together and creating an evil entity, then went on to abuse."

Ahead of their trial, Fred died by suicide and Rose was found guilty and convicted in 1995 of 10 murders. She is currently serving her life sentence at HM Prison New Hall – where she continues to maintain her innocence.

Fred and Rose West: A British Horror Story is available to watch on Netflix now. Sign up for Netflix from £5.99 a month. Netflix is also available on Sky Glass and Virgin Media Stream.

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