AlanM:
Aircraft at the base of CAS at 2450ft with Mode C can be overflown at 3000ft legally as they are deemed to be outside CAS
Indeed. But it's worse than that. Aircraft with no Mode anything - primary returns only - can be directly overflown even though the controller has no information whatsoever on whether the aircraft concerned is at 6 ft, 66,000 ft, or anywhere in between. They're also deemed to be below controlled airspace. I've always thought it's an entirely bonkers practice, but it's done every day of the week at Heathrow and everywhere else, and of course these airports would be closed down if you didn't allow this "deeming".
to be fair, it is more pragmatic now, in terms of 'loss of separation' stats
Not sure what you mean by that - care to elaborate?
NS