In reply to BusyLizzie:
One of the minimalist Salomon vests that carry 2 * 500ml soft flasks on the shoulder straps, so you can drink from them without removing them from the pockets - very comfortable, hardly know it's there, and the soft flasks collapse as you drink from them, so there is a lot less "sloshing" compared to chest mounted bottles.
Great storage pockets on front, side and back. They come with varying amount of storage ( over and above the flasks ) from 1L to 12 L. Not cheap though !!
eg
http://www.salomon.com/uk/product/s-lab-sense-ultra-set.html has 3 litres additional storage which covers food, waterproof, spare base layer.