Bookworm, I trust that you meant that the discussion on CTRs should stop there and not the whole thread, cos I would like to come back to the discussion between Woss and Chilli.
The fact of the matter is that no such collision has taken place in UK airspace since the war. There is no better argument on risk than the actuarial one.
Some ATCOs and and Authorities seem to treat a "loss of separation" as seriously as a collision. This is like my wife's reaction to losing a child in the park. Just because you can't see him doesn't necessarily mean that he has been abducted or run-over, just as loss of separation doesn't mean the worst has happened.
I still feel safer in the class G corridor between BIG and OCK in a 2000' cloudbase at 2300' than I do at 1900'...and I believe that feeling of safety to be justified by the realities of the situation.
Will