Last appearance of Hugo Speer as Inspector Valentine; first appearance of Tom Chambers as Inspector Sullivan.
This episode references "priest holes". During the time of Catholic persecution in England, many of the larger manor homes owned by Catholic families were outfitted with an intricately designed secret hiding space specifically so that Catholic priests could be hidden from royal troops who intended to hang members of the Catholic clergy. Priests were hidden in spaces behind walls and under floors and due to the cleverness of the deception, no one today knows how many of these priest holes still exist.
The dates on Elspeth's grave marker say that she died in 1944. She disappeared 9 years before this episode was set, so we can firmly date the episode as taking place in 1953.
Father Brown is asked if he is a wizard. He is a wizard in the Harry Potter movies.