Serves me right for getting annoyed about someone not willing to take the advice of those who really would be trying to help
I am curious at astonishing the leap of imagination it has taken to reach the above "conclusion" from my entirely neutral technical discussion of radar blip size / azimuth accuracy. Nobody was criticizing Shawbury, who are unfailingly polite and helpful.
1 degree means 0.5nm at 30nm, which is obviously close enough to detect a real CAS bust.
I would agree that a radar system will be more reliable than GPS (which is why I would take a radar vectored letdown over a GPS approach) but there is no doubt that GPS is far more accurate in the 99.xxx% that it is working.
With a decent moving map, either displaying the actual CAA chart (this is now possible) or used together with the printed chart, it's quite hard to bust airspace.
The only time to my knowledge I have busted UK airspace was a vertical bust, when flying, at about FL053, under a piece of FL055-base Class A and then transiting to below an adjacent piece of Class A with a 5500ft base. This type of "let's see how many pilots we can catch with this trick" airspace design seems peculiar to the UK. The QNH was significantly off 1013 and this resulted in a bust of about 500ft. I was accompanied by an ATPL instructor, who was watching a moving map of the actual CAA chart, and he didn't spot it either... it won't take long to work out where this was but it was a long time ago and the tapes will be long gone. It was in the middle of the night and no radar service was available to us.