Old film stock would need processing. Perhaps it was filmed on a cine camera? If it was video it might need editing. The fact it went on at all (and on BBC premises) is disgusting.
IF and that's a big if it went on, if the source is an ex employee I don't see them having anything to lose being named.
How come the source knows soooo much? Were they making it all up for the money, if the incidents happened were they party to them, could they themselves be a paedophile, has the newspaper pasted the name of the source to the police for further investigation?