I don't do camping;-P but the weekend before last (Lammas) there were people camping just outside the henge, and camper vans parked on the village car-park all night. I know it's not 'allowed', but it seems if you are cheeky enough you can camp anywhere. Who's going to stop you?
In the early nineties, at the height of 'crusty' . . there used to be people camping all over. . by Silbury, at the Sanctuary.

I stopped at the New Inn at Winterbourne Monkton. Caught the bus into Avebury (every hour on the half hour) and walked back (about a mile). en suit bathroom and a full english breakfast next morning! £25 - Job's a good 'un!