In reply to ianlaw:
The one I am thinking of is a field at the corner opposite the village of Morvah. We didn't actually stay there, just left our car there overnight while we walked the coast path. It's very basic, with a toilet cubicle (you can see these on the aerial photo, by the hedge), and there were just a few people camping there when we turned up. I can't believe it would ever be booked up, or even that you would normally book.
Sorry, but I can't find the name of it. There must have been a sign at the gate with a phone number though, because we phoned to ask if it would be OK to park, and left some money in the bucket for the things for cleaning the toilet. They were very friendly on the phone.
Post edited at 13:52