Mrs Lydia Harsnet was a non-canonical character who appeared in the 2010 Agatha Christie's Marple adaptation of the novel The Pale Horse. Mrs Harsnet is the widow of the former vicar of Much Deeping. After her husband died, she was offered a position as the housekeeper of Captain and Mrs Cottam. During the events in the episode, she is staying at the Pale Horse along with the Cottams while repairs are being made to the Cottam's house which had been damaged by fire.
Lydia "discovered" the body of Captain Cottam. At first it was supposed that he had died of natural causes while taking a nap. However during the denouement, it was revealed that Cottam had died from a poisoned love potion made of Spanish fly. Lydia had been having an affair with Cottam. She purchased (regularly, it was suggested) a love potion from Bella Webb, the "local witch" and a member of the staff at the inn. Unknown to Lydia, Cottam (and even Bella), the potion had been poisoned. Cottam consumed the potion while in bed with Lydia and died while making love to her. Lydia apologised to Mrs Cottam for her affiar. She explained that Captain Cottam insisted on it and that she thought Kanga knew about it.
Lydia Harsnet is portrayed by Sarah Alexander.