Don't forget Welshpool - there's a top quality campsite in walking distance (about 15mins). Out the airfield gates turn right and follow the road over the Seven, then make a left and the campsite is on the left.
I dont think Skeggy are happy with camping under the wing since they moved the parking area as it attacts other campers into that area. They were against it when I asked a couple of years ago, but certainly worth a call to be sure.