In reply to John Aisthorpe:
Refuge du Bolet de Satan in Recloses, quite central and cheap (12€), accommodates 15 people (maybe you can ask them to accommodate one more):
http://refuge-du-bolet-de-satan.overblog.com/ A friend of mine went there with his uni club and I think they enjoyed it. Seems like you can camp too for 7€. I popped in there and it looked decent, I seem to remember there's even some big chimney where you can cook (in addition to the stoves obviously). The "manager" may not speak English very well (at all?) but is very talkative and seemed like a nice chap.