In this episode, it's said one of the suspects has first-degree burns over half his body. His burns are shown as disfiguring and he is said to have been in hospital for over a year. These would be third-degree burns. First-degree burns are the least-serious category of burns.
About 20-25 minutes in, Det Supt Sandra Pullman (Amanda Redman) is interviewing David Swallow (Colin Tierney), who was badly burned in the fire they're investigating. Tierney's character has severe scarring to the right side of his face. Redman's character is wearing a short-sleeved top, revealing severe scarring to her left arm. Tierney's scarring is a prosthetic, applied by the make-up department, Redman's scarring is real, caused by being burned by a pan of boiling soup when she was 18 months old.
Jack mentions the serial killer, Dr. Harold Shipman. James Bolam played Shipman in Harold Shipman: Doctor Death (2002).