So, as title says, I need a cheapish 20 litre or thereabouts backpack.
It's for work, so I can have a waterproof coat, a drink, a bit of food and pens, etc in.

Basically a version of a womans handbag, but bigger, for a fat, sweaty balding British guy.

Any recommendations? Less than £30 ideally - it doesn't need to be particularly strong etc. And ideally not in camouflage colours.