In reply to Darren:
Not sure which car park you mean by "behind the campsite". Is that the National Trust campsite just past the lake or the other campsite just before the pub?
If you mean the one near the pub and Barn Door Shop then just before you get to it is a large grassy area on the right, triangular in shape, where you can park for free without any restrictions, its officially the village green. There's also a public toilet maintained by the local community about 100 yards back down the road (i.e. before you get to the green).
The National Trust car park and campsite is about a mile before the green over a bridge on the right as you drive up the valley.
Zoom in on this and you can see both car parks labelled, the campsite near the pub isn't labelled but you can see tents in the field.
https://www.google.co.uk/maps/search/wasdale+village+green/@54.4633976,-3.2...
Post edited at 18:56