Mountain Warehouse have a sale on, including some rucksacs that have a mesh backplate on them to raise the bag off your back & let air circulate to keep you cool & not get sweaty, etc.
There are 20L, 30L & 35L sizes;
http://www.mountainwarehouse.com/Bags_Rucksacks/Day_Packs/sc1402/p193.aspx
http://www.mountainwarehouse.com/Bags_Rucksacks/Day_Packs/sc1402/p69.aspx
The bonus is that you can get another 10% off voucher emailed to you by filling in a customer survey here;
https://www.mountainwarehouse.com/customer_survey/store/index.aspx
I got the 20L and 30L ones from a local shop and use them for commuting. The smaller one is ok if leaving shoes in the office, 30L fits everything in no probs.
They're both good on the bike, are light, don't interfere with helmets & the rain covers are handy for winter.
Thought this might be useful to someone….
