In reply to Beano:
Northumberland is huge and wild looking with not that many people around. I would guess, pick any wild/semi-wild location and use common sense (i.e no multi-day base camp, fire, big groups etc etc and you will be fine.
I have managed to wild camp in places like Norfolk, Northamptonshire, Shropshire etc along river banks and never had an issue ever. I imagine from the times I have been to Northumberland I could go missing for several days and not be found camping up there.
Enjoy