In reply to top cat:
i've no idea if you could lay your hands on one, but Berghaus used to do the Extrem Exped which was about 80l - it was a climbing sac, rather than a comfy walking sac, but it was perfectly acceptable and very hardwearing.
as mentioned, the Berghaus Vulcan or Cyclops is a good option if you can live with green, they are more comfortable to carry for long trips than the Extrem Exped, and last forever. mine (Vulcan) is 20 years old, and has from the begining been ridiculously overloaded, badly treated and is still in reasonably good nick.
if colour really isn't important, but cheap and functional is, the current issue PLCE long back infantry sac is less comfortable than the Vulcan, prety much as strong, very practical and can be had for £40 - 50.
Post edited at 12:54