Moors Murders: The Witness | Preview (Channel 4)

Moors Murders tells the story of Britain's most infamous child murders - by 1960s serial killers Ian Brady and Myra Hindley.

The three-part series is built around never-before-seen prison letters from Brady and Hindley to the same source, written over the five decades after their capture. Expert analysis of these letters provides new insight into the killers' backgrounds, thinking and motivation.

The series asks why both murderers - especially Hindley - remain our most notorious embodiments of evil. This first episode features an eyewitness account of the last gruesome murder, from Myra Hindley's teen brother-in-law David Smith.

Smith alerted the police and brought the spate of murders to an end. Sixties pop singer Janie Jones befriended Hindley in prison, later conducting a 40-year-long correspondence with both Hindley and Brady. She explains how Hindley minimised her role in the murders and pinned the blame on Brady and Smith.

Her correspondence casts new light on Hindley and Brady's relationship. Forensic psychologist Kerry Daynes and criminologist Professor David Wilson analyse the letters. The letters reveal for the first time Brady and Hindley's intended next victims.

Journalist Clive Entwistle tells how he met Myra Hindley within hours of the last murder and describes how the police investigation began: culminating in the shocking discovery of a tape recording of a young girl being tortured - a first indication that the police were not dealing with a routine murder, but the most notorious of serial child killers.

The complete Moors Murders series will be available to watch on All 4 after this episode is broadcast.