In reply to robert mirfin: People camp/campervan in an open field alongside a river further up the valley (vague I know but you can't miss it if you drive on from Kilnsey). Unless this has changed - I've not been up there for a while.
Locals will know more.
As for campsites, they seem to be universally overcrowded on weekends and the owners have a *bad attitude*. Is this a theme in Yorkshire?