While serving her sentence at Low Newton Prison, the real Anne Darwin was occasionally mistaken, by the other inmates, for serial killer Rosemary West, who was also detained at the prison. This was due to their similar age and the fact that they both wore glasses. Monica Dolan, who plays Anne Darwin, previously played Rose West in Appropriate Adult (2011).
The houses that they used to portray the Darwin's homes, and where John enters the sea, are not in Seaton Carew but further up the coast at the Headland, Hartlepool.
John's canoe has "orca" written on it. This is the name of the boat in the movie Jaws.
The main character in this film, and also the main character in "Stonehouse," were both inspired by the same plot point in the Frederick Forsyth novel/movie "Day of the Jackal."