Lidl will have a selection of hiking rucksacks instore from Monday 07/04/14 for £16.99, they look pretty good quality as well.
Product Information :-
Hard-wearing, tear-resistant Ripstop outer material
Air-Stream System for optimal air circulation
Ergonomically shaped straps with comfortable mesh padding
With a range of compartments as well as loops and fastening hooks for hiking poles and other accessories
Includes protective rain cover
Capacity (L): 35 or 40