I too have bought a smaller rucksack, this one from Mountain Warehouse
http://www.mountainw...-bag-p2131.aspx.
It is actually a hydration sack but they have relabelled it as a bike rucksack. I have found it very comfortable and hardly notice i am wearing it. Its quite amazing what you fit in it, i have a spare tube, tool kit, lights, pump, light weight waterproof and extra warm gear. Also has useful pockets. Its also long enough to hold a d lock. Because its long, fits very close to body so doesn't swing around so don't use fitted waist belt.
Its obviously a good colour for cycling as well some reflective detail. Generally i have found their gear well made for the price, no i am not going to start a thread on effectiveness of reflective gear
.
Forgot to add, as to not getting a sweaty back due to some fancy design, rubbish. Just have to accept it.