In reply to Godwin:
I looked at this question quite abit recently and gathered this information on weight. 100g of weight is definitely not as important as design, build quality and durability but hope it's useful
dmm zenith 18l 580g
blue ice dragonfly 18L 415g / 325g stripped
petzl bug 18L 510g
bd rock blitz 15L 403g
latok 20L 480g
simond alpinism 22L 530g (quite alot can be stripped off)
simond climbing backpack 20L 360g
I think the foldable/packable bags are a good option to pair with a bigger 'approach bag' as they are tiny (45g), I've ordered one but haven't tested it out yet. I've seen these type of bags are used quite often in winter to carry some kit/comforts on the route.
Adding in a chest strap (<£5) is worthwhile for climbing. The fabric is very lightweight so definitely not durable, that doesn't matter much for the cost of a <£10 bag - but there is a fair concern about losing it's contents on a thrutchy route.