The Firkins have always denied any involvement in the murders
Lawyers for two Somerset men jailed for killing a Cornish couple have been denied the right to question a key prosecution witness during an appeal.
Brothers Robert and Lee Firkins were jailed for life in 2006 for murdering Graham and Carole Fisher in 2003.
They were beaten with a sledgehammer and shot at their garage in Wadebridge.
George Carter-Stephenson QC, for Robert Firkins, said a witness who testified against the brothers at the trial lacked "credibility".
Three appeal court judges declined a request to call "witness X" to testify about his reasons for writing a letter in December 2006 to a newspaper reporter which defence lawyers claim casts doubt over his testimony at their trial.
The witness, who is on a protection scheme and cannot be identified, had shared a cell with Robert Firkins.
He told the Exeter Crown Court trial in January 2006 that his cellmate had confessed to killing the couple at the petrol station they owned.
Carol and Graham Fisher were killed on Bonfire Night
At the start of the appeal hearing, Mr Carter-Stephenson urged Lord Justice Hughes, Mr Justice Treacy and Sir Peter Cresswell to allow the witness to be recalled.
But after legal argument Lord Justice Hughes refused the request saying there was "no basis for doing so".
"There has been the fullest investigation into his letter, and there's no reason to suppose that the letter or anything said about it raises doubt about the truthfulness of his evidence," the judge told the court.
Unable to cross-examine the man about his letter and possible motivation for writing it, Mr Carter-Stephenson argued that the trial judge should have dismissed the case against the brothers because of the unreliable "cellmate confession".
Mr Carter-Stephenson told the appeal hearing that the witness had been "motivated by reward" and that there was "no starting-point for credibility" in relation to his testimony.
Robert Firkins, 35, and Lee Firkins, 33, have always denied their involvement in the murder of Mr Fisher, 60, and his 53-year-old wife.
The pair, from Weston-super-Mare, claimed they were seeing relatives and had visited a local pub when the attack took place.
The hearing continues.