The first Sarah Jane

The answer to one of Doctor Who's oldest mysteries has been provided on the DVD release of Invasion of the Dinosaurs.

It has long been known that Elisabeth Sladen was not the first choice of actress to play the role of Sarah Jane Smith. For years the identity of the original actress has remained a close secret, known only to members of the production team.

However, the production notes on the new DVD release, compiled by David Brunt, have revealed that the actress first selected to play the role was April Walker. Brunt discovered the name on a stray memo from the BBC files, informing the BBC Finance department to pay the actress when each of the eleventh season stories went into studio.

Walker had a similar pedigree to Sladen at the time Sarah Jane was cast, with small roles in a number of television series including Crossroads, Dad's Army and The Onedin Line.

Walker was cast in 1973 as the Doctor's assistant by then-producer Barry Letts, who was looking for a replacement for Katy Manning, who had left the series at the end of the previous season. Contracts were signed but when Walker met the current Doctor, Jon Pertwee, it became apparent there was a lack of chemistry between the two. Although the pair had worked together on stage about five years previously and got on quite well, she was not the sort of actress Pertwee wanted to play the companion. He felt she was too tall, busty and too much of a dominant performer. Not the sort of tiny girl he could "protect" as the Doctor.

For actors to be replaced after they have been cast is not common, but not unknown. Sometimes the face just doesn't fit and however good the actor is, the production team realise that a replacement is necessary. Letts took the decision to recast the part and eventually selected Sladen, this time with approval from Pertwee. To take the decision to recast the role was a very difficult one for Letts and one he never discussed in public. Walker was paid in full for the Eleventh season.

Following her brief encounter with the Doctor, Walker has gone on to have a long career with over 40 appearances listed on the Internet Movie Database. She is perhaps most well known for playing Jean in the 1975 Fawlty Towers episode The Wedding Party. She also had roles in Anna Karenina, The Two Ronnies, Wyatt's Watchdogs and Minder.

The forthcoming third issue of the Doctor Who production research fanzine Nothing at the End of the Lane will contain Walker's first ever published interview concerning her casting and replacement in Doctor Who. The fanzine is released later this month.