And this has got me interested, because...what previously unseen footage is there? There's a few outtakes that have been doing the rounds since they snuck into the infamous BBC Christmas tapes, but that's not really unseen, and I'm not aware of any studio material that's survived (could be wrong, but I'm sure if it existed it would be known about, and probably have been used, in much the same way as, say, the studio tape of Death to the Daleks). Could be some home movie footage taken by cast or crew, I guess.
Or maybe, it could have something to do with the rumoured thirteenth episode?
ETA: Ah, and here's a report from digitalspy. "Unseen footage that hit the cutting room floor." Should I be this optimistic?