The credits for the 1984 Cannon Films adaptation of the novel Ordeal by Innocence list a character named Martha Jessup. Exactly who she is is a mystery as there is no such person in the show. She was presumably in a scene which was subsequently removed during post production editing. One possibility is she is one of the middle-aged women whom Jacko Argyle chatted up in order to cheat them of their money. In Chapter 20 of the original novel, Arthur Calgary interviews one such woman (whose name is not given). Martha Jessup might have been in such a scene.
Martha Jessup is portrayed by Anita Carey.