DD Hammocks classic one is only £27 and it's a double layer one (
http://www.ddhammocks.com/product/camping-hammock?from_cat=32), but you've also got ENO and thermarest. Or just about any one in a similar fabric that's long and wide enough. You can even make your own, just search for DIY hammock.
The important thing is to realise that the hammock itself is just a small part of the setup. Think of it as the inner tent, but comfier, you wouldn't go camping with nothing but the inner tent. You will need insulation underneath, a cell foam mat or partially inflated thermarest works well too start with (sleeping in a sleeping bag only and you will experience CBS - cold butt syndrome, as the filling is compressed between you and the hammock). You will also need a tarp for wind and rain protection- a long enough basha on a paracord ridge line is fine.
Finally the suspension can be as basic or as high-tech as you want it to be. But as long as you use tree huggers to protect the bark and remember that you'll generate a lot of force so don't skimp on strength, you'll be good.