I bought a nice big backpack on recommendation from someone in this forum. Has loads of compartments and fits everything i need for cycling in it so no need for fitting anything to my bike at all. Even with all my locks, spare inner tube, a laptop (macbook, pointing this out not for elitism but more to point out it's got an aluminium case so weighs a TON), some fruit, a 2 litre camelpak and a change of clothes (can't fit shoes in with all that though) because it's an excellent backpack it doesn't feel that heavy.
Not cheap though and rare as hell:
http://www.wiggle.co.uk/vaude-bike-alpin-air-25-5-rucksack/
I'm sure there's plenty of other brands that do just as good a job too though.