I'd go for Inov8 too - they have a selection of capacities depending what you want to carry. In mine, the bladder fits at the waist part of the pack which seems to help the fit. Also, its really adjustible with no annoying flapping straps!
Well that seems to have split opinions somewhat! Actually I will feel AMAZING if I achieve under two hours and if others think this is not "maximising my potential" that seems a shame. Achieving a personal target and enjoying the journey was my main motivation and its encouraging that many see it the same and that it is within my grasp if I up my game without going mad. I love the comment about enjoying the event, the scenery and the comapny of other runners - it sums up my feelings about entering Keswick Half. Thanks to everyone who posted.