In reply to malc:
Try www.bluebellfarmbelford.co.uk they have some camping plus bunk house and some wooden huts for hire. In Belford Village on the road out towards Bowden Doors, 2 minutes from shop and chippy and 4 minutes from the pub (i.e. a near perfect location).
Alternatively there is a much larger campsite near the beach at Warren Mill which is about 2 or 3 miles from Belford on the BB1342 Belford to Bamburgh Road.
http://www.budlebaycampsite.co.uk/touring-camping/
There is another site about half a mile beyond Warren Mill towards Bamburgh.