In the sixth and final episode of Death of a Codebreaker, Dr Sian Williams asks what justice for Gareth Williams would look like. She hears from Jim Frazer, the forensic scientist who carried out a review into the case 11 years ago as well Angela Gallop who has reviewed dozens of cold cases over the years. She asks if advances in DNA and scientific techniques could help detectives finally solve the mystery of how Gareth Williams died. And she questions whether it's time to put the security forces under greater scrutiny to reveal more about the nature of Gareth's work with M16.

Death of a Codebreaker is presented by Dr Sian Williams with contributions from Dyfan Rees, Ashley Byrne, Wayne Wright, James Pepper, Tracy Cardwell and Lewis Allsopp

The series was produced by Wayne Wright, Ashley Byrne and James Pepper

The music is by Robin Forrest and the sound design by Kevin Bailey and Mark Burrows

Executive Producer: Ashley Byrne

Death of a Codebreaker is an MIM Production for BBC Sounds and BBC Radio Wales