One of Doctor Who's oldest mysteries has today been answered! Some of you may or not know that Elisabeth Sladen wasn't the first choice to play Sarah Jane, and she actually replaced the original actress very near to filming starting on Doctor Who's eleventh series.

For many years, it's been a mystery who this woman was, but it has finally been revealed to have been April Walker. Her name was discovered when David Brunt was checking the BBC files for the Production Notes of the DVD release of Invasion of the Dinosaurs and found her name on an old memo.

The story goes that Barry Letts was looking for a replacement for Katy Manning, who was leaving the series, when Lets met April Walker. She soon after signed contracts to become the new companion, but it soon appeared it wasn't going to work out as Jon Pertwee thought they had no chemistry together (and he apparently said she was 'too tall', 'too busty' and 'too dominant').

Barry Letts then found Elisabeth Sladen and the rest is history. Jon Pertwee gave him his thumbs up, and she became the most popular companion in the history of the show. There are so many 'what-ifs' surrounding this story. What if April Walker became Sarah Jane? What would she have been like? Would she have had a similar impact? Would she still be so relevant today? Most likely not...

April Walker never broke through after Doctor Who, and instead have small roles in several shows such as Faulty Towers, Two RonniesMorecambe & Wise Show and Waking The Dead.

Watch the video below (skip to 4.40) to see April Walker playing the character of Jean from a 1975 episode of Fawlty Towers - The Wedding Party

Next mystery, just what planet is Tom Baker from?