In the novel The Mirror Crack'd from Side to Side, Longdon's is the draper in St. Mary Mead. Miss Marple asked Miss Knight to go there and see if a pair of curtains were ready, among other errands. This was actually part of a ploy to get Miss Knight out of the way so that she could leave the house and explore "The Development" undisturbed.
When Miss Knight got to the drapers, the curtains were, of course not ready. Miss Knight said she knew it all along but made the enquiry anyway to humour Miss Marple. She thought Miss Marple was "failing a little". The senior assistant at the shop who knew Miss Marple better remarked after Miss Knight had left that it was nonsense. Miss Marple was still "sharp as a needle".