For what they are worth, my thoughts;
I attended the BGP for many years in the 90's. Parking was always free, but in the post along with the entrance tickets came a parking permit, which had to be displayed on the windscreen. The fact that the parking was free was irrelivent, all that the permit showed was that you had bought entrance tickets and had every right to use the car parks for free. Kev did state that not every car had a fixed penalty notice on it. Did these cars, or some of them at least, belong to people who did not go to the race and were only there to steal hubcaps or whatever. A lot of us know from bitter experience that a car park during such an event is an ideal place for skulldugery. If Silverstone still send out parking permits as they used to, and ask that they be displayed then surely they have every right to place a fixed penalty notice on cars not doing so as in theory they have no right to be there.