Thanks for the input.

I've read good things about this bag on various outdoors forums:

http://www.amazon.co.uk/Highlander-E...pr_product_top

I'm also looking at the idea of two bags. You can get a Decathlon bag which packs small and is lightweight, and could fit into another bag - though I have heard some warning about this practice. Apparently it can compress the filling of the bags and reduce their effectiveness.

I'm also looking at Army surplus (unissued) bags. This one seems okay, in that it isn't overly heavy or bulky (well, compared to some of the other ones).