So now we know that the legendary background source during Watergate known as Deep Throat was FBI No. 2 man Mark Felt. And so ends one of the most compelling mysteries of our time. Now all that’s left is one final puzzling conundrum from the same era as Watergate and Deep Throat. It’s just as seemingly insoluble, and no one who knows is talking.

I am speaking of course of the 30 year-old mystery of who was originally cast as Sarah Jane Smith.

We all know the story. Barry Letts cast an actress, initial work was done with her and then…she was dropped because she ‘wasn’t working out’. (Rumours abound that this means anything from she was terrible to she didn’t get along with Jon Pertwee to she got along a little too well with Jon Pertwee…) The identity of the actress has never been revealed, in spite of the fact that Doctor Who fandom has some of the best researchers on the planet. (They can find 40 year-old telesnaps but ask them to find an employment memo from 1973…hmph). Heck, I once tried to get the answer out of Terrance Dicks while we were both in a cab and very drunk, but he insisted that he didn’t know.

Even this year at the Gallifrey One convention, Barry Letts refused to name names, though he offered one tantalizing hint: the actress was bought out of her contract, which would have meant that she was paid for the entire season. This would have required approval from the Head of Serials and memos and such…and yet this has eluded our crack team of researchers.

Rather like determining the identity of Deep Throat there are some interesting theories. One theory goes that it was actress Fiona Gaunt, who played Helen in Letts and Dicks more adult (read: boring) SF series, Moonbase 3.  Moonbase 3 and Season 11 of Doctor Who were being produced around about the same time, and being moved over to Moonbase 3 would mean that she didn’t necessarily get bought out of her contract per se, hence why no paper trail has emerged. And she was a dreadful actress (Moonbase 3 all but ended her television career).

My favourite theory was one that was conceived in a fit of silliness at Gallifrey last year: that they cast someone who’s now very famous. I mean, if you were Barry Letts would you really want to admit that back in 1973 you had originally cast Judi Dench as Sarah Jane Smith but sacked her?

Whatever the case, it’s nice to know that Doctor Who can continue to provide interesting real-world mysteries to speculate endlessly about.