Who's in Caerphily Castle?

So we were driving home last night and found lots of big red trucks parked on the green by the Castle. Some looked like mobile libraries, some like the vampire truck that comes around to take your blood. Aside from a small catering van, none were marked and all were fenced off with security guards patrolling.
Then across the moat and actually by the Castle, more vans - but smaller. Kind of box wagon things.
Feeling the need to investigate, because I am a nosey bugger, we had a pootle nearer to see if we can figure out whats going on.
Of the plentitude (is that a word?) of cars parked in the accompanying road, three had CBBC passes hanging from the rear view mirrors.
So a BBC production, in South Wales, where CBBC are also involved, but cant be Sarah-Jane as that's has wrapped ... can you say Doctor Who?
Needless to say, you cant near the sodding place ad it would appear that filming is taking place within the grounds of the castle itself too .. but I shall return tonight and is stuff is still going on, I shall endeavour to get pictures.
Just be prepared to look at fences and trucks......