The last time I was there the little lay-bys by the northern henge were sporting 'No Parking' signs. I still parked there, but I got the impression that someone might take umbridge if I'd been there for any length of time...

There is a camp site just down the road from the henges. I think they charge 15 quid or so a night, for two people in one tent.

http://www.sleningfordwatermill.co.uk/index.html

Cheers
Dave