I was at the Gly Y Mul farm site last weekend. The 20 acre woodland is down by the river and there are a handful of excellent spots besides the river where you can can either pitch a tent or a tarp/hammock set up. There are lots of other area's a 'stones throw' from the river but you may end up up setting up in amongst the bracken and brambles. Be warned though that getting along the river bank area if its been raining requires boots or wellies as it gets very muddy.
That aside though, once you park your transport its only a couple of minutes, over the railway line (two gates) and down to the river. Though it rained for 50% of my time there, the constant sound of the river, the occaisional blast of sunlight and the once only sighting of a kingfisher, made for a very enjoyable and peaceful weekend.
Furthermore, back at the main site (if you use it) there are toilets, sinks, showers and drinking water. Apart from making use of the porcelain every morning i stuck down in the woods, the owner Ian, leaves you to get on with whatever you want, and he encourages fires.One word about the fires, there was not a great deal of standing deadwood around where i was so you may have to travel a bit to get some, or Ian sells logs i believe.
Overall, a £4.00 a night per person its a great place to go.