The savage murders of Carol and Graham Fisher, who were shot and bludgeoned to death at their home in North Cornwall on Bonfire Night 2003, shocked the nation.
Brothers Robert and Lee Firkins, from Weston-super-Mare, now in their late 30s, were convicted and jailed for life at Exeter Crown Court in January 2006. They were ordered to serve a minimum of 26 years.
Lawyers representing the pair, who consistently denied the murders, confirmed yesterday they are preparing a fresh bid to the Criminal Cases Review Commission (CCRC).
It follows the murder conviction of a man – known only as Witness X – who claimed Robert Firkins confessed to the killings while they shared a cell.
It is claimed Firkins bragged: "Watch Crimewatch and you'll see my work."
Solicitor Jane Hickman, who represents Lee Firkins, told the Western Morning News: "We will definitely be taking the case back to the CCRC. But for his evidence there would have been no conviction."
Ms Hickman said she regarded the conviction as "completely unsafe" and described Witness X as a "dangerous, violent and manipulating individual".
She added: "One of the reasons Witness X gave for giving his evidence was that he had discovered God during his last stay in prison and that having seen the light he wanted to make amends.
"We said at the time that was not believable and that he was motivated by the reward which was being offered by Crimestoppers."
The extreme violence of the Fishers' murders shocked police and unnerved the local community.
Mr Fisher, 60, was found dead from three shotgun wounds in the kitchen of their home near Wadebridge.
His 53-year-old wife was first shot in the hand, and left a trail of blood through their bungalow before being shot in the back and the neck as she tried to flee down the garden path.
The couple died from massive head injuries caused by blows from a sledgehammer.
Their bodies were found the next morning at their garage business on the A39, near the Royal Cornwall showground.
Carol's brother Nigel Edwards, who lives in Falmouth, said the family were "shocked" at news of a possible further appeal.
"All we have ever wanted is justice for Carol and Graham and we thought we had that," Mr Edwards told the WMN. "We are in shock and I felt physically sick when I first heard. It is like we are living the nightmare all over again."
The reliability of Witness X's evidence, and its importance to the case, has been previously tested at the Court of Appeal and the European Court of Human Rights.
After a four-day hearing at London's Appeal Court in 2008, the brother's appeals were dismissed by three top judges: Lord Justice Hughes, sitting with Mr Justice Treacy and Sir Peter Cresswell.
Lord Justice Hughes said that although, as a "dishonest" man, it was possible that the witness' evidence contained "embroidery", there was more evidence to back it up.
"There were a great many criticisms to be made of X," the judge said.
"His character was bad. He was a recidivist criminal, largely committing acquisitive offences, but also with a drug habit.
"There had been significant violence towards a former girlfriend. In prison, there were repeated reports of drug dealing and 'taxing' of other prisoners.
"He was undoubtedly a dishonest man and he was demonstrated to have lied at a number of points in his evidence when these and other aspects of his history were put to him."
But Lord Justice Hughes added: "This was not a case which depended entirely on Witness X.
"Whether or not the other evidence would have justified a conviction in his absence, he certainly did not stand alone."
While denying the murders, the brothers had admitted a number of offences during a spree of violence in 2003.
In November, Robert Firkins attacked three men in the Grenville Arms pub in Nanpean, near St Austell – spraying two with ammonia.
Two days later, he and his brother went to Clacton, Essex, and caused actual bodily harm to a man called John Garrett while in possession of a shotgun.
On December 19, Lee Firkins fired a shotgun during a masked robbery on a Jet service station at Fraddon in Cornwall. The next day, Robert Firkins used a knife to slash the face of a man at his home in St Dennis. He was also squirted with ammonia before he was bound with tape and dumped in the countryside.
The next day, the brothers robbed the B&Q hardware store in Taunton, Somerset, dropping all but £600 of their £1,800 haul.
In 2011, Lee Firkin took his case to the European Court of Human Rights in Strasbourg, arguing his right to a fair trial had been breached.
His legal team said the jury of "non-lawyers" could not be expected to understand fine legal points, particularly the strictly limited relevance of the "cell confession".
However, the court's Registrar President, Lawrence Garlicki, rejected Firkin's complaints, saying they were "manifestly ill-founded".
