Sounds like a good route agileman. I'm afraid I can't help you with campsites as March was waaaayyy too cold to be contemplating a tent. The b and b in kielder mentioned that the midges are dreadful in the summer.
One lesser known highlight of the reivers is the microbrewery at Hesket Newmarket.... I stayed at the B and b next door, so I'd check the map for suitable sites around there!