Cast changes: Due to the mid-series departure of Sara Martins (Camille), Joséphine Jobert (Florence) is promoted to Detective Sergeant (DS), and Tobi Bakare is introduced as Officer Hooper to maintain staffing of two officers, a DS and the DI.
Character Duncan Roberts objects more than once to people using his old nickname "Disco Biscuit". The term is a drug reference - in context most likely referring to MDMA (aka Molly, or Meth) - other drugs have been given the name at other times since the 1960s. The character has moved the furthest from his old rock & roll lifestyle to respectability and wants to live down that aspect of his past.