BD, it's not a "campsite" campsite, if you see what I mean. It's a field (from which the sheep have hopefully been removed) with a small loo shed in one corner. As you're stood facing the 3 Stags, the camping field is the field next to it on the right - there's a gate about 5-10 yards along from the Stags. It should be easy enough to find though, on account of there being a bunch of tents in there already!
The owners are pretty relaxed about it all. If you arrive late, just pay them in the morning.