I use this: http://www.tesco.com/direct/tesco-hydration-rucksack-blue-6l/750-9905.prd?skuId=750-9905
It used to be £7, and it has lasted for 3 years of near continuous use running and cycling.
Its not waterproof, and if you can't leave shoes at work then you're screwed. However, if you use a plastic bag to protect stuff from wetness, and only need to carry shirt, lunch and trousers (and underwear and socks), its pretty incredible.
I use this:
http://www.tesco.com/direct/tesco-hydration-rucksack-blue-6l/750-9905.prd?skuId=750-9905
It used to be £7, and it has lasted for 3 years of near continuous use running and cycling.
Its not waterproof, and if you can't leave shoes at work then you're screwed. However, if you use a plastic bag to protect stuff from wetness, and only need to carry shirt, lunch and trousers (and underwear and socks), its pretty incredible.
Plus its really good for trail runs.