I got myself a Berghaus (20l free flow II)from Millets that was under £40. Apparently the branch in Kingston has quite a collection of bags. Took me ages to find one I liked and I spent quite a bit on cheaper ones before I discovered one that was just right. I had ones that made my back sweaty (so get a concurve back), chafed under my arms (being a lady with ...er..appendages in places where men don't have them, the straps cut me at funny angles across the chest); rode up my back, jiggles around etc etc. There are good websites to do some homework re prices and styles but the best bet is to go and try one on and see how it fits when you trot around the shop!! My back is also quit e short in proportion to legs so had to check out length too. Bag choosing became quite a mission!!!