In reply to UKC News:
I was climbing Calculus Crack at the time, on the Apron to the right of the North Wall and saw the whole thing come off. The YouTube video shows it pretty well (taken by a group of guys just ahead of us on the same climb). Totally terrifying, especially as that area is pretty popular for it's long-difficult routes.
Police said that everyone who was climbing in that area has been accounted for.