Thanks all for your replies - apologies; I read them and then never replied.
I have a bike with a rack and some panniers for more serious loads, this is to be a bag for the "I''m off out for a ride" bike. I think the SQR system looks best for me given the lack of loops on my saddle, and so now it's a question of which size bag - do I want room for a stove and coffee-making stuff as well as the camera, waterproofs etc just because sitting in the countryside having a a brew is nice, or do I want the smallest one I can get away for 90% of the time for neatness?
Or do I just accept that I'm going to end up with more than one at some point? :-)