April Walker

 was the original actor cast and contracted for Sarah Jane Smith. She was quickly replaced by Barry Letts because of a lack of chemistry with Jon Pertwee. Nevertheless, she was paid in full for the whole of season 11, as her contract demanded. Her identity was a closely-guarded secret, and something which neither Barry Letts nor Elisabeth Sladen revealed in their lifetimes. It was only uncovered by info text writer, David Brunt who happened to chance across an order for her payment in the course of research for the Invasion of the Dinosaurs DVD. (INFO: Invasion of the Dinosaurs)