POLICE have said that they believe that a missing teenager, Lee Boxell, who disappeared more than 30 years ago after visiting an unofficial church youth club, was killed after he witnessed a sexual assault.

A new appeal concerning the disappearance of the 15-year-old was broadcast last Friday on BBC1, on Crimewatch Roadshow Live, in which the Metropolitan Police said that they now believed that he was killed shortly after he disappeared.

He was last seen in Sutton High Street on 10 September 1988. Police initially thought that he was on his way to watch a football match, but new evidence suggests that he attended an unofficial youth club in an outbuilding at St Dunstan’s, Church Lane, Cheam, which was known locally as “the Shed”.

William Lambert, a graveyard digger at St Dunstan’s who ran the Shed, was imprisoned in 2011 for sexually abusing four girls. After his conviction, it was revealed that Lambert, a former soldier, had been found not guilty in 1994 of raping girls at the same youth club, and had been found guilty of gross indecency against a child in 1968.

One of those he abused described him as “a Svengali character who held initiation ceremonies to pass on the next level of his ‘witch powers’ — and then sexually abused us”.

Lambert, then aged 78, and two other men, aged 52 and 41, were arrested in 2014 on suspicion of murdering Lee. A woman was arrested on suspicion of conspiracy to pervert the course of justice and indecency with children. They were all released without charge.

Police have issued a new appeal urging others who may have been abused by Lambert to come forward.

In a statement, the Metropolitan Police said: “Detectives believe that Lee witnessed someone, probably a teenager, being sexually assaulted. It is believed that he either intervened to stop that assault or threatened to report it.

“Detectives also believe that he was subjected to an assault in order to stop him from identifying and exposing the suspect, and that this assault proved fatal.”

Police spent a year excavating the churchyard at St Dunstan’s, but nothing was found.

They are now appealing to the victim of the sexual assault to tell them what happened, and those who assisted in removing Lee’s body to come forward, “before it is too late for Lee’s family”.

A spokesperson for the diocese of Southwark said: “We join the police in urging anyone who can help with the enquiries to contact them.

“The parish of St Dunstan, Cheam, is keeping all those involved in their prayers.”