April Walker

 was the original actor cast and contracted for Sarah Jane Smith. Barry Letts quickly replaced her 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 neither Barry Letts nor Elisabeth Sladen revealed in their lifetimes. It was only uncovered by info text writer, David Brunt who happened across an order for her payment in the course of research on the Invasion of the Dinosaurs DVD. (INFO: Invasion of the Dinosaurs)