The unmarried homosexual had earlier been cautioned by police for "helping" two young teenagers make an indecent video in the bathroom of his home, Norwich Crown Court was told .
While Roberts was in jail awaiting trial he told two inmates he considered Thomas a "rent boy" and that he hated him. Thomas's family yesterday strongly denied allegations that Thomas was involved with sex and drugs.
Judge Mr Justice Smedley told Roberts: "You have been found guilty by the jury of strangling to death a young boy of 12. You described him to a witness as 'not an angel'. I have no doubt he was not - very few 12- year-olds are - but he didn't deserve to die in the appalling way you treated him."
Thomas disappeared after cycling from his home in Happisburgh, Norfolk, on the evening of 21 August 1997. His body was found next day, 50 miles away near Thetford, in a picnic area used by gay men.
He had been strangled with his black shoelace necklace and black round- necked T-shirt. Other marks suggested he may have been sexually abused.
The jury was told that two years before the murder, police had cautioned Roberts over the obscene video he had helped two teenagers make. The boys had also watched some of Roberts' collection of pornographic videos, that included titles such as Teen Sex and Action Boys.
Roberts was born in north-west London, and moved to Eccles in the mid- 1990s so he could be near his sister.
Thomas had become a regular visitor to Roberts' Granville's Stores on the Bush estate in the summer of 1997. But it took a year to link Roberts with his murder. Charles Wide, QC, prosecuting, said a breakthrough had stemmed from Thomas's broken shoelace necklace, found lying on his shoulder with two tiny beads missing .
A matching bead was found in the U-bend under a sink in Roberts's bungalow. In a pounds 2m investigation, officers travelled to Hong Kong to trace the beads. They also monitored Roberts' movements and discovered he was having an affair with his cousin, Peter Roberts, manager of a dairy in Thetford, near where the murdered boy's body was left.
A former neighbour of Roberts told the court she saw the shopkeeper in his garden standing over an upturned mountain bike around the time Thomas vanished.
The boy's parents, John and Carol Marshall, yesterday said Roberts had misrepresented their child in court.
Mr Marshall, who is in his early 50s, said: "He had a stable and loving family. We did not permit him to smoke, and do not believe his references to drugs were any more than adolescent bravado. He was generally considerate and polite, although at times cheeky, like any 12-year-old boy."Reuse content