We have stayed at Cavalier Fields three times now. It used to be listed on the Silverstone web site camping page, but got knocked off last year.
It is a couple of miles from the circuit (Luffield corner), but we take bikes and cycle in. We also like the Fox & Hounds pub in Whittlebury, which is on the way to the circuit.
Cavalier Fields is quiet, well drained, friendly and not as expensive as closer camp sites, but still has a good atmosphere. Don’t go there if you want to be camped on top of your neighbour and party all night (please!)
We went to the Barcelona GP for the first time this year and had a fantastic race. We sat in grandstand ‘G’ (between turns 12 and 13) and could see the cars for 20% of their entire lap time! You can see the circuit map here.