Miss Marple does not appear in Agatha Christie's original novel, which was an adventure featuring her other sleuths, Tommy and Tuppence Beresford. In order to make the story part of the Marple series, the time period was moved from post-war to sometime near the end of the war. This conveniently places Tommy still in the military intelligence service abroad, and his part of the story was re-written for Miss Marple.
Although the date of is not made clear, it appears to be set in the Post-War late 40s or early 50s. The movie version of Jane Eyre (1944) that it refers to was made during the war in 1944, so although the poster art from the film appears similar to the Robert Stevenson film, it doesn't mention either Joan Fontaine or Orson Welles, who starred in the film. The part of Jane's friend, who dies in the film, was originally played by Elizabeth Taylor, although she dies from pneumonia, not leukemia, as stated in "By the Pricking of My Thumbs."