Think yourself lucky - I've driven on farmers land before & had a shot gun produced from the back of a Land Rover..!!
Logically speaking, to put a charge on parking would necessitate some form of signage & publicity. If there was neither of these things, somethings very much amiss.
I'm guessing the farmer gave the field over to parking & hired the parking firm, who should have done the signing etc.
May be worth a call to Silverstone to find out if this was the case & which farmer?
Of course it may be a practical joke from someone else parked in the field..
