The Devil You Know, Part 2 - S4.E2

Eric Thornton and John Littlewood figure prominently in hospital files, but when it's confirmed that Littlewood has died, the police focus all their effort on Thornton. A dark history of mental problems and violence related to them, Thornton stands out as the prime suspect in the girl's murder, but when his own mother (Celia Imrie) is murdered in cold blood, James isn't convinced that Thornton is responsible for either death.