Fixer, thanks for fixing the link. Seems like only 99p for another 1/2 lt bladder although the Tesco one is immediately available I suppose. I have a Gelert, paid more than that though unfortunately, but there's room for front and rear tubes, puncture repair kit, waterproof (nearly) overgloves and other stuff in the rucksack itself.
I think that a back pack is overkill. It's good to take water but I've never drained my 1.5ltr pack and I cvan carry the same. I'm going to cut down on what I carry as the more weight you carry makes it harder to 'jink' about on the bike.
__________________
I do know where I'm going, but the track it will decide, It's not the destination, It's the glory of the ride!