...I might go to Decathlon at the weekend.
decathlon do a series of 20/30/40 ltr sacks that are surprisingly well made given their price tag (between £17 and £40), i've got a 30ltr one that just use as a casual daysac for work and play i'm pleasently surprised by how comfortable it is and how well its put up with the crap i give it.
now, to be clear, the design, materials, build quality etc.. are
not of the same standard you'd get in POD sac, or an Osprey, or a Berghaus. if i'm going for a walk over the Malvern hills or the Black Mountains in Spring/Summer/Autumn, or taking the kids for a picnic on the Stiperstones then i'll use it - if i need a sac to get rammed with full winter gear with a good chance of some abrasion then i'll use something else, the same goes for a bothy trip with the weight of stuff i'd take. the crux of whether the 40ltr one would do you for short backpacking trips is quite how lightweight you consider lightweight to be...
personally i'd have a feel of the decatholon stuff, then pop to Cotswolds or somewhere and have a feel of their offerings - if ypu're happy then great, save £60 or so and go Decathlon. if not then remember the maxim - buy cheap, buy twice.
(oh yeah, remember that different manufacturers have different ways of working out capacity - so two 40 ltr sacks may have very different capacities. my Decathlon 30ltr sack is perhaps 15% smaller in capacity than my 30ltr Berghaus Munro..)
