The Doctor From Gallifrey 

It has long been known that Elisabeth Sladen was not the actress who was originally set to play Sarah Jane Smith but she refused to tell the world who that person was. The news was revealed on the new DVD release of "Invasion of the Dinosaurs" and the original actress was April Walker.

Walker is probably best known these days for her guest role in Fawlty Towers as Jean Walker in the episode "The Wedding Party". Walker worked in rehearseals for The Time Warrior but allegedly the pairing of Pertwee's Doctor and Walker's Sarah Jane didn't work: she was a tall and more obviously strong character, along the lines of Pertwee's first companion, Liz Shaw (Caroline John).

It's believed that Pertwee was unhappy with the decision but it would ultimately have been producer Barry Letts who recast the role. Walker was reportedly paid for both The Time Warrior and the rest of that series.

Who knows what could have been? Maybe in a parallel universe there is a woman called Jean Walker starring in The Sarah Jane Adventures. For us though, Elisabeth Sladen was the perfect Sarah Jane.