Look away immediately if you don't want to be spoiled regarding future Doctor Who episodes. I have no idea how much this ruins about the finale to David Tennant's era as the Doctor, so to be on the safe side I advise you only read this if you're really, really sure that you don't mind discovering a plot secret almost eight months ahead of broadcast.
You have been warned.
Elisabeth Sladen and Tommy Knight recorded scenes yesterday for the forthcoming Christmas and New Year's Doctor Who specials - which conclude with the Tenth Doctor regenerating into Matt Smith's Eleventh. David Tennant was present, and had a brief scene with Knight (last time their characters interacted was via computer screen - the actors never met).
John Simm's Master was also thought to be present.
Bear in mind that this adventure airs after The Sarah Jane Adventures' third season has concluded on British TV - if the traditional September through to December slot keeps - so this will likely be set after that. The implication means that Sarah Jane and Luke survive the events of series 3 (as if they wouldn't...)
Master filming spy - and former SJ.tv reporter - Scooty has a collection of images from filming.
Russell T Davies is interviewed in today's Telegraph, and states that "It's not quite as easy to guess what's happening as you think - there's nightmare sequences, and layers of fantasy, because the Doctor's coming to the end of his time. It's quite interesting to watch things being filmed, and think: 'Oh, I can see what that would look like...'" So, is this scene everything it first appears?
(The clapperboard confirms it's for Doctor Who, and definitely not the SJA before you ask...)
More on this exciting story when the news comes in to us.
(Oh, and here's my own little crazy theory about "he will knock four times", if you wanna hear. Go ahead and knock twice in very quick succession on the desk infront of you - sounds uncannily like a human heart beat. Now, imagine four knocks - that'd be twice as many heartbeats; the same as a man with two hearts would produce...)