I was looking for a medium size rucksack for my hols when I spotted this at Sports Direct. You may or may not be a fan of the company but eitherway it's a pretty good price.
The same pack seems to retail for around £60+ elsewhere.
"This 65 litre Karrimor Bobcat 65 rucksack has lots of great features that are great for a few day treks.
Supercool back system, Grab handle for easy transportation, Compatible with a hydration system, Side compression straps, 2 compartments
Removable sit mat, Key clip, Reinforced with bartacks and rot proof thread, Progress fixed side pockets, Pocket in lid for additional storage, Front map pocket
2 mesh wand pockets, Under-lid compression strap, Base load straps & chest straps, Lid shock cord carrying system
Ice axe/walking pole attachment
Size: 65 Litres"