Detectives are hoping new forensic evidence may finally solve the two-decade old mystery of a couple's execution-style murder.

The fresh investigation into the killing of Peter and Gwenda Dixon on the coastline of west Wales is being carried out by Dyfed Powys police force, who today revealed that "interesting initial forensic results" have already been established.

They have launched Operation Ottawa into the deaths in July 1989 of Mr and Mrs Dixon, who lived at Moorland Road, Witney, and were on a camping holiday.

The couple were shot at close range with a shotgun. Their bodies had been dragged from a coastal footpath and dumped in a wooded gully near cliffs.

The same specialist forensics team is also looking into another unsolved case, of siblings Richard and Helen Thomas, who were similarly murdered with a shotgun four years before at nearby Milford Haven. Nobody has ever been charged in either case.

For the past two years the team has painstakingly recovered, cataloged and recorded everything from the investigations. DNA techniques, not available at the time, could yield new lines of inquiry.

The man in charge, Det Chief Supt Steve Wilkins, said: "There has been considerable success nationally, by adopting a forensic review approach to cold case investigations.

"Material which 20 years ago would not have been considered suitable for forensic recovery is now routinely submitted to forensic science providers and has proved successful in identifying DNA profiles, resulting in the arrest and conviction of offenders for serious crime.

"At this stage some interesting initial forensics results have been returned on these cases, but it is too early to put these into the context of other evidence and further tests need to be carried out."

The couple had a son Tim, now in his 40s, and daughter Julie, in her 30s and a former student of West Oxfordshire College in Witney. They now have their own families and do not want to comment on the latest police initiative.

But Keith Dixon, Peter's youngest brother who lives in the Reading area, said Mr and Mrs Dixon's children were devastated by their deaths.

He said: "To lose a parent is a painful experience but when a parent dies with the family around perhaps there is a dignity to that, but they were robbed of their parents and that is something you do not come to terms with. The impact is with you for the rest of your life, it leaves a mark.

"We would like somebody brought to account. It would not make our loss any better or easier but it would carry a sense of closure.

"They say grief subsides with time and it does to a degree, but I think the impact of losing close family members in these circumstances does not go away. It is as painful and relevant today as it was 18 years ago."

Mr Dixon, who lives in Tadley, Hampshire, said his brother was a man of integrity and character, and added: "I miss him, I miss them both."

He added that the couple loved the Pembrokeshire coastline and had been going there for 20 years, even though they could have afforded to go anywhere in the world.