The London Underground Killer description:

In a South London Police Station, officers are shocked to discover a man has been murdered in a locked cell. The suspected killer is his cellmate, a homeless man called Keiran Kelly, in custody for theft. Police interview 53 year old Kelly, who has a long rap sheet for petty crimes, and he calmly confesses to killing the man in the cell, but detectives are unprepared for what comes next.