The Order of the Garter dates from 1348, and James was made a member in 1590. The motto ("Honi soit qui mal y pense") is displayed on a small belt ("garter") around the left calf as an insignia of office. As of 2018, over a thousand men and women have been made members and the order is still active.
The word "Satan" is spoken 39 times during "The Witchfinders". In comparison, Series 2's The Satan Pit (2006) only uses the word once (excluding the title).
The title "The Witchfinders" parodies the film Witchfinder General (1968), based on the life of obscure historical figure Matthew Hopkins. Vincent Price, the star of that film, wanted to make and star in an American film adaptation of Doctor Who (1963) in the early 1970s, but was unable to secure the rights. The "Witchfinder General" concept was also spoofed in the BBC's Good Omens (2019) (based on a 1990 novel), which starred Tenth Doctor David Tennant, who was also Jodie Whittaker's Broadchurch (2013) co-star.