Thanks all, you have confirmed what I initially thought but I do prefer to ask the experts before making a decision.
@Wombat, unfortunately I am still compiling our gear so don't have the all the gear to do that at the moment, not to mention it probably won't all fit in the bag I do use.
@vghikers, I tried the 33 on, unfortunately only with padding and no real weight in Go Outdoors, but it felt super comfortable, and as @ninthace said, the pole stowage system was what made me decide an Osprey was for me.
@Strider, I am actually entertaining the thought of using a back pack for my springer but he's so nuts he'll probably roll all over the place with it still on. We used to have a bullmastiff who came everywhere with us especially when going camping. We used a backpack on him and he was great carrying his poop bags and extra water, right up until the day we decided to put some shopping in there. Right in the middle of the walk back, he decided to roll on his back, there were broken eggs everywhere, squashed grapes, all sorts, it was so bad we had to throw the rucksack away and take him straight for a shower!