Have a look at this one. Its No.7 on the independents list of the best campsites in the world. Never stayed there myself but am planning to at some point.
GWERN GOF ISAF FARM, CONWY
“Scenery of the very tall, very rocky variety soars skywards from the edges of the site wherever you cast your amazed gaze,” says Andy. Gwern Gôf Isaf is situated on a traditional north Wales hill farm at the bottom of the looming Tryfan peak. Set in the heart of the mountains, it is a great base for walking or climbing the surrounding peaks of the Glyders and the Carneddau. The National Mountaineering Centre at Plas-y-Brenin is only three miles away.
Where: Gwern Gôf Isaf Farm, Capel Curig, Bettws-y-Coed, Conwy (01690 720276; http://www.gwerngofisaf.co.uk)
When: all year
How much: £3 per person per night