In reply to jobertalot:
Based on our experience a couple of years ago, most places around there tend to be a bit Caravan Club, book-online-six-months-in-advance. We stopped at a site in Belford, which was a bit manicured but reasonably relaxed with it and had semi-tame rabbits everywhere, and the place at Once Brewed on Hadrian's Wall, which was pretty basic and good. I believe that the rugby club in Alnwick does camping over summer as well, which is meant to be fairly cheap and cheerful.