Lady Margaret Mayfield was a non-canonical character who was created for the film adaptation of The Incredible Theft in Season 1 of ITV's Agatha Christie's Poirot. Lady Margaret Mayfield was the daughter of an earl (from whom she had her title) and the wife of industrialist and arms manufacturer Tommy Mayfield.
Early in the episode, Lady Mayfield called Poirot anonymously, and then later as "Mrs Smith", wanting to meet him. They meet at London Zoo but she annoyed Poirot because she disregarded the rendezvous instructions. Poirot had been carrying a copy of the Times and she was supposed to wear a green carnation but she did not.
Lady Mayfield told Poirot that her husband had invited Mrs Vanderlyn to spend the weekend at their place. At first Poirot was disinclined to get involved but Lady Mayfield persuaded him. It was more than a domestic problem, but one of national security. She told Poirot about Mrs Vanderlyn's suspected role in foreign espionage. There was the case of the Marquess of Lowestoft for example. He committed suicide when implicated in passing state secrets to Mrs Vanderlyn. His regiment had been involved in the testing of the "Galahad tank". Persuaded by Lady Mayfield, Poirot agreed to take on the case.
Lady Mayfield was played in the film by Ciaran Madden. This character does not occur in the original story. The character there who fills the same roles of Tommy Mayfield was the slightly different Lord Charles Mayfield who was not married.