April Walker

 was the original actor cast and contracted for Sarah Jane Smith. After an arduous audition process, Barry Letts formally awarded her the role on 14 March 1973. However, he had not involved Jon Pertwee in the process and realised, too late, that Pertwee didn't approve of the choice. Letts therefore quickly replaced her with Elisabeth Sladen, someone who had not been involved in the auditions that produced Walker. (INFO: Invasion of the Dinosaurs)

Although she had been effectively fired, Walker was reportedly 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 foe the Invasion of the Dinosaurs DVD.

Walker was not pleased with her dismissal. By way of apology, Letts would cast — and actually use — her in his 1975 production of The Prince and the Pauper. (INFO: Invasion of the Dinosaurs)

Outside her near-career in Doctor Who, Walker has enjoyed a long career in British television, that has stretched across more than forty years. Her body of work tends towards sitcoms, where she has mostly had memorable guest-starring roles. She has been a part of the key guest starring cast of some of the best-loved British comedies, like The Two Ronnies, Dad's Army, Fawlty Towers, and Yes Minister.