My brain is too dead for me to enter serious technical discussions! However, most of my life spent using radar within the London Control Zone and many years using handheld GPS for car navigation and outdoor hobby work, plus more recent acquisition of SBS, all give me reason to trust radar above GPS any day. Our in-car nav is based on the popular Memory Map programme, versions of which are used by some pilots. It is usually extremely accurate but just occasionally it does strange things. Travelling along a very straight section of the M3 recently it tracked us about a quarter of a mile to one side for several miles and then regained the road. It's a road we have driven many times and it is usually accurate. We have experienced this before but have no explanation for why it happens.
I know little about modern avionics but understand that aircraft ADS-B transmissions (which provide lat and long info on a/c position) are derived from some form of GPS equipment. Watching these transmissions on SBS is very interesting and it is not unusual to observe position errors of 2nm or more. A few days ago I saw one of at least 5nm.
All of this leads me to have very considerably more confidence in ground radar than current GPS derived information and I am sure that if radar observed a pilot infringing an airspace boundary then he really did. Having dealt with many, many such incidents whilst at Heathrow I would just add that some pilots would argue black was white when accused of such action!