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

Young Welshman Gareth Williams was found naked inside a relatively small bag in his bath in 2010. And another theory is that he was the victim of a bizarre sex game gone wrong. There are suggestions he may have had an interest in Claustrophilia - pleasure derived from being in a confined space. But how much evidence is there? Here Dr Sian Williams looks at what we know and don't know about Gareth's private life.

Death of a Codebreaker is presented by Dr Sian Williams with contributions in Episode 5 from James Pepper, Ashley Byrne, Tracy Cardwell and Wayne Wright.

With so many unanswered questions, different theories about what exactly happened to young Welshman Gareth Williams in summer 2010 remain rife. One suggestion is that he was assassinated by a rogue state. A former KGB intelligence officer, an ex-Government Minister and the wife of murdered dissident Alexander Litvinenko all suspect Russian involvement. But would the Kremlin really go to such lengths?

Death of a Codebreaker is presented by Dr Sian Williams with contributions in Episode 4 from from Dyfan Rees, Ashley Byrne and Wayne Wright.

The inquest into codebreaker Gareth Williams' death threw up many still-unanswered questions about the circumstances surrounding how he died. Here, Dr Sian Williams examines more closely some of the evidence that came to light during the eight-day hearing and, given the nature of the young Welshman's work for the security services, asks if we will ever know some of the most crucial detail! Journalist and executive producer Ashley Byrne joins Sian as they work through some of the evidence from the hearing with a series of guests with an interest in the case.

Death of a Codebreaker is presented by Dr Sian Williams with contributions in Episode 3 from from Ashley Byrne and Dyfan Rees

Young Welshman Gareth Williams was a child prodigy who became a talented mathematician and codebreaker working for GCHQ, MI6 and the intelligence services but in the summer of 2010 he was found dead in a bag in the bath at his apartment in London. In this first episode Sian Williams looks at what we know about Gareth and reminds listeners of the bizarre nature and circumstances of his death.

Death of a Codebreaker is presented by Dr Sian Williams with contributions by Dyfan Rees and Tracy Cardwell.

From the outset, one big question has haunted detectives looking into the death of young Welshman Gareth Williams. How could his body have ended up locked inside a relatively small bag in the bath at his apartment in Central London?! In this episode Dr Sian Williams hears from two people who have attempted to lock themselves inside a similar size holdall but with altogether different outcomes

Death of a Codebreaker is presented by Dr Sian Williams with contributions from Lewis Allsop, Wayne Wright and Dyfan Rees.

Dr Sian Williams leads a team charting the story of the Welsh codebreaker whose naked and decomposing body was found inside a bag in his bath at

Mystery still surrounds how and why Gareth Williams, from Anglesey, died - not least how he ended up inside the North Face sports bag in the first place. There were no fingerprints on the bag, on the padlock or in the bath.

Gareth’s death continues to baffle police with speculation as to the cause ranging from a sex game gone wrong to involvement of a rogue state, possibly Russia.

The coroner Fiona Wilcox, who led the inquest into the 31 year old’s death, gave a narrative verdict, concluding it was probable Gareth had been

Gareth Williams normally worked for GCHQ in Cheltenham but had been seconded to the Secret Intelligence Service at MI6 in London. It’s still unknown what the exact nature of his work was and evidence by the secret intelligence service at his inquest was minimal.

Gareth Williams had not been seen by family or work colleagues for over a week when his body was found and it was only when his sister raised the alarm that MI6 contacted police who carried out a ‘welfare call’ at his home in Pimlico. When officers entered his flat they found the heating was on full, (which was unusual for the middle of August) and this may have caused issues for pathologists investigating exactly how Gareth died.

In this programme Sian and the team dig deeper into the story.

We hear conflicting evidence from experts who attempted up to 300 times to lock themselves inside a North Face sports bag.

We delve into some of the evidence not reported widely from the inquest into Gareth’s death, we hear more about what his work at MI6 may have actually entailed and we find out some more about Gareth the child genius who went on to be recruited by the intelligence services because of his brilliant mind as a mathematician and codebreaker.

Experts give their views on what may have happened to him. We delve some more into the various theories including possible Russian involvement - and we ask whether 13 years on, with notable improvements in DNA gathering procedures, if it’s time to re-open the investigation into his death and to finally bring closure for the family and justice for Gareth.

Among those Sian and the team meet over the series – Marina Litvinenko, widow of murdered Russian dissident Alexander Litvinenko and former KGB agents as well as people who have worked for the British intelligence services and senior police officers who oversaw the enquiry into Gareth’s death.

Death of A Codebreaker is presented by Sian Williams with contributions from Dyfan Rees, Tracy Cardwell, Ashley Byrne, Wayne Wright and Lewis