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Scrutinising a decades old Bullseye show Steve Wilkins realised he had the piece of evidence that could nail a psychopath police had been hunting for years.

Shotgun killer John Cooper had been suspected since the early 1990s of being behind the murders of Richard Thomas, Helen Thomas and holidaymakers Peter and Gwenda Dixon.

Now in an explosive new book the retired detective chief superintendent has revealed how he and ITV presenter Jonathan Hill found the footage that led to the serial killer’s conviction.

Mr Wilkins had asked the journalist for his help after inquiries revealed Cooper had appeared on a 1989 episode of comic Jim Bowen’s darts quiz. After days of searching a librarian found the episode in Leeds. It was February 2009.

“When Jonathan called me I could hardly believe what I was hearing,” Mr Wilkins said.

“He said that the episode with Cooper had been recorded on 28 May, 1989, exactly a month before the Dixons were murdered.”

Cooper executed the middle-aged Oxfordshire couple with his shotgun as they strolled on the Pembrokeshire coastal path near Little Haven on June 29, 1989. He robbed Mr Dixon and sexually assaulted his wife.

Four years earlier he had shot dead millionaire farmer Richard Thomas, 58, and his sister Helen, 56, at their Scoveston Manor home near Milford Haven. That was on December 22, 1985.

When Jonathan Hill called Mr Wilkins about their breakthrough he could hardly contain himself.

“It’s incredible Steve,” the broadcaster told him. “You have to see this.”

When he did, the investigator was astonished.

“It was Cooper, bold as brass, smiling and joking on national television having almost certainly murdered two people in cold blood,” he said.

Host Jim Bowen chatted with Cooper and the other contestants before the game started.

“The conversation was relaxed as the contestants talked about their lives and playing darts,” Mr Wilkins said.

Bowen asked Cooper how he spent his spare time.

“You’ve got an unusual hobby John, haven't you?” the comic said.

“Oh yes,” said the killer. “The scuba diving.”

Bowen remarked Pembrokeshire was “the place to do it”.

“We’ve got deep water where you can swim over mountains and all sorts of things,” Cooper said.

Mr Wilkins said: “You could hardly make it up. “There was Cooper, just a month before the killings, revealing on national television knowledge of the area where the Dixons would be murdered.”

Cooper let himself down in the quiz. His female opponent was faster on the buzzer and had the right answers.

“Cooper forced a smile but he would have hated losing, especially to a woman,” Mr Wilkins said. “In the end Cooper and his teammate were forced into an early exit.”

But when his opponents declined the chance to gamble their winnings for a shot at the star prize, Cooper was given the chance.

“He had been thrown a lifeline,” Mr Wilkins said. “Having been humiliated on national TV, here was his chance to redeem himself and scoop a big money prize.”

Grinning Cooper strode back on set. He wanted to risk his winnings for the star prize.

“It was another extraordinary chance to scrutinise the prime suspect,”Mr Wilkins said. “This time it was Cooper throwing the darts and not his partner.”

The murderer threw his three darts and lost the lot.

“He looked to camera with a stare that said it all,” Mr Wilkins said.

“As the credits rolled the host tried to console the pair with a friendly hug and a joke.

“One can only imagine what Cooper was thinking.”

The footage was now a key piece of evidence.

“We had the prime suspect on tape just a month before the Dixons were murdered. His physique, haircut and features could all be scrutinised.”

In a Cardiff edit suite Mr Hill viewed the artist’s impression of the suspect from the 1989 inquiry. The sketch captured the suspect from behind his

right shoulder. His face was almost in profile.

“Watching the Bullseye sequence through, Jonathan then stopped the tape at the moment when Cooper adopted a similar pose,” Mr Wilkins said.

“The images were then put side by side with the help of a graphic artist who pasted the still images next to one another on the screen.”

The result was chilling.

“Cooper had long, bushy collar length hair just like the artist’s impression,” Mr Wilkins said.

“Given that Cooper’s hair is likely to have been brushed into shape by a makeup artist before the TV show, it matched closely. His features too bore a close resemblance, particularly the nose and chin.”

Then there was his stocky physique. It was just like the man in the sketch. The suspect was drawn with his shirt and shorts pulled tightly across his body. The similarity – now spotted more than 20 years after the crime – was compelling.

“For the first time we could see Cooper as he would have looked at the time of the Dixons’ murder,” Mr Wilkins said.

“In my 30 years service I had seen many artist’s impressions and photo-fit efforts, but I had never seen as close a match as this.”

(Image: Wales News Service)

Cooper was jailed for life in May 2011 for the two double murders.

A Swansea Crown Court jury also convicted him of sticking five terrified teenagers up at gunpoint in woods at Milford Haven’s Mount Estate, of raping a 16-year-old girl and indecently assaulting another aged 15.

Cooper has since been implicated in five other murders.

His appeal was rejected in November 2012.

He will never be freed from jail.

The Pembrokeshire Murders: Catching the Bullseye Killer, by Steve Wilkins with Jonathan Hill is published by Seren, priced £9.99