In reply to pcassels:
> It needs to be large enough to fit 2 sleeping bags, stove, food for 2 and general walking kit...
yes, your walking partner gets a bigger pack and/or fitter!
Unless you have lightweight (expensive) gear otherwise, and especially if you have synthetic sleeping bags, then 80+ litres likely to be needed.
Kate is quite right. Pop to your local independent store for good advice.
Osprey packs of that size are commonly available; you missed the half price Gregory Baltoro sacks over at SportPursuit unfortunately.
b