The Investigator: A British Crime Story

In the fourth and final episode, a former police officer who investigated the original case tells Mark he believes it is plausible that Russell could have burned Carole's body as a way to dispose of it as he claimed in his confession letter.

Mark learns police recently visited Russell in prison to hold a meeting with him. Sam's son Neil contacts Mark to update him on developments. Neil says: "He [Russell] stands by what he said in the letter. The letter's completely truthful. Meyrick Park golf course was one of the locations he's given as where he scattered the ashes."

Mark heads to the golf course to see whether he can find any evidence, 30 years on.

Russell identified a broad area in his garden where he claims to have burnt his wife's body before scattering her ashes at different locations around the country, so Mark instigates a dig, and asks a specialist in cremated remains to investigate the claim. Sam has a brief recollection of the fire in June 1985. She says: "When I came home, my father was stood up on the bank with the fire … My father was really, really angry with me. And he ripped my dress and he sent me into the home."

Also, more letters arrive from Russell which add a further twist as the series reaches its end.

Teenager Louise Kay has been missing since June 1988 in a case that has baffled police and left her family desperate for answers.

Williams-Thomas's investigation sends him on the trail of the prime suspect in her murder, convicted killer Peter Tobin, who was living nearby at the time. As he delves deeper into the case he discovers a series of other missing women and unsolved murder cases who could also be the undetected victims of a serial killer.

In the second episode, Williams-Thomas investigates the notorious murders of Anna Kenny, Agnes Cooney and Hilda McAuley - who were all abducted and brutally killed in similar circumstances within four months of each other in Glasgow in 1977.

He investigates the prime suspect, convicted killer Angus Sinclair, and the evidence suggesting he is responsible for the unsolved murders.

Mark Williams-Thomas's investigation uncovers another Glasgow murder that bears all the hallmarks of suspected serial killer Angus Sinclair. There is just one problem - unlike the murders of Anna Kenny, Agnes Cooney and Hilda McAuley, this crime has already been solved. A week after 37-year-old baker Frances Barker's body was discovered, lorry driver Thomas Ross Young was arrested, and was subsequently jailed for life. The investigator tries to establish whether the police got the wrong man - or if there could be another explanation. Last in the series.