Mark Masefield, who was friends with victim, claims he killed Pryke in self-defence during row and hid the body in a drain

The best friend of a teenager whose body was discovered in a drain more than two decades ago will this week be sentenced for manslaughter after he contacted police and confessed to killing his friend.

Mark Masefield, 43, contacted police in the West Midlands last July and admitted being responsible for the death of his friend Anthony Pryke, who went missing from his home in Chelmsley Wood, Solihull, in August 1987, aged 17.

His body was found four months later by workmen investigating a blocked drain, but his remains were too decomposed to be able to establish a cause of death.

Masefield told officers he hit Anthony over the head with a scaffolding pole during an argument. He is due to be sentenced at Birmingham crown court for manslaughter on Thursday, after previously admitting the charge during a court hearing.

When the teenager disappeared from his home in 1987, it was widely assumed he'd left the area as Pryke's father had died some months earlier. But workers called to the empty house to clear a blocked drain in December that year found his body.

Anthony's brother, David, and a second man were later arrested and charged with his murder, but they were both released without charge and the police investigation drew to a halt for a number of years. A coroner in 1988 recorded an open verdict at his inquest as it was thought Pryke may have been overcome by fumes while hiding in the drain.

It was only when David Pryke approached police in 2011 to say he had learned Mark Masefield had made a drunken confession to a friend that the case was reopened. A cold case review was started but the friend who'd heard Masefield's confession chose not co-operate with police.

The victim's body had been cremated and exhibits, notes and witness statements from the original police investigation were destroyed after a flood at Chelmsley Wood police station in the early 1990s. For some reason, Masefield contacted officers in July last year and indicated he was ready to confess.

In a statement, West Midlands police said: "This remarkable case has always remained open and last year was subject to intensive scrutiny by West Midlands police's major investigation review team, which specialises in investigating historic and cold cases.

"It goes to show that we do keep unsolved crimes open; reviewing them on a regular basis and doing everything we can to bring offenders to justice. The last 25 years have been incredibly traumatic for Anthony's family and I hope that when these court proceedings come to an end it will bring a sense of closure for them and allow them to move on with their lives."

Victoria Jones, a senior prosecutor with West Midlands CPS's complex casework unit, said: "Masefield revealed he'd had a row with Anthony and he'd smashed a scaffold pole over his head. He confirmed he had pushed Anthony's body down a manhole in the garden. He claimed it had been in self-defence, that Anthony had gone to attack him and he'd retaliated."

When Masefield was interviewed in 1987, he'd provided a false statement claiming he had not seen his friend for some time, since his father's funeral. The sentencing will take place at Birmingham crown court on Thursday.