Whitstable Ernest is a non-canonical character who was created for the 2009 ITV adaptation of They do it with Mirrors. He is one of the inmates at the Stoneygates reform institution who had a record of financial crimes. He turned out to be an expert at fiddling with accounts and Lewis Serrocold used him to help embezzle funds from the Gulbrandsen Foundation. When Lewis saw that the trustees were beginning to suspect something, Ernest was used to set fired to Carrie Louise's study in an attempt to destroy the financial records there.
Later, he was found severely ill after eating oysters which smelled rotten. However Miss Marple surmised that Ernest came from the town of Whitstable which was famous for its oysters. He would know if oysters smelt or tasted off and would not have eaten them. Thus it must have been that Lewis Serrocold had supplied Ernest with the oysters and whiskey as a bait and laced the oysters with arsenic to poison him to prevent him from talking. Lewis had noticed that Johnnie Restarick was becoming friendly with Ernest and at some time might be able to elicit the truth from him.
Whitstable Ernest is played by Jordan Long.