Oh, and another thing when testing (although you might feel a bit of a wally) is put something heavy in them and bend over as if you were on the bike. Many 15l-25l sacks seem to be more "bags with straps" and you don't get the more technical fittings or the wide waist belts of the 40l+ sacks.Thanks Norm, very informative and useful. There is a Go Outdoors store around the corner so will use that to establish comfortable style etc.
Air-flow backs (many companies do similar) are more worthwhile on a bike, IMO, as you are moving faster so the air gets a better chance to circulate.