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.