In reply to neilh:
It might be better value to get full size ones that will do them for the next decade or so. Vango have always made decent bags - Nitestar used to be the decent entry level bags, although they are a bit heavy for expeditions - and I've been using one of their 3 season Ultralite bags for the last 10 years. But you'll find a decent range at £60-90 in most shops (Karrimor, Mountain Equipment, etc.), and they probably all have similar weight and performance.