Went to north Norfolk, place called Burnham Thorpe, where the horses had paddocks and you could sleep in a tent (or your lorry if you preferred). Can't remember the name of the farm offhand but it had a website so you could google it.
Little Lodge Farm, Thetford in Norfolk - it's fantastic there. Loads and loads of off road riding and a lovely river to cool off in. And if your feeling brave, a cross country course! I have been 3 times now and am off again in June!
You can sleep in tent, or horse box. Or if your flush, they have a couple of lovely log cabins and mobile homes.