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 his flat in Central London in August 2010.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 unlawfully killed but because there was no clear evidence she couldn’t deliver an official verdict of unlawful killing.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 Allsopp. The series is produced by Wayne Wright. Executive Producer is Ashley Byrne Research by Wayne Wright, Ashley Byrne, Maya Mitter, Kurt Brookes, Andrew Edwards and Tracy Cardwell. Music by Robin Forrest Sound editing by Mark Burrows Death of a Code Breaker is an MIM Production for BBC Sounds and BBC Radio Wales.