Mrs Sadler was a non-canonical character who was created for the film adaptation of The Third Floor Flat in Series 1 of ITV's Agatha Christie's Poirot. She was a stage character in the play "The Deadly Shroud" at Wyndham's Theatre. Poirot was feeling miserable from not having had a case for three weeks and suffering from a bad cold. Hastings took Poirot to watch the play, betting him ten pounds that he could not guess who the murderer was,
Mrs Sadler was played by Gillain Bailey (billed as Gillian Bush Bailey). There is no such character in the original story.