Thanks for all the comments.
I've already got the wildcat harness, so not really looking to replace.
At the moment I use an 7L rear seatbag, a fuel tank, a 10l bag in the harness and an osprey pack with odds and ends and water in it. I've not got a frame bag.
I'd like to lose the backpack, hence looking at the gourdon, which would solve the water carrying and have loads of room for everything I need (way too much, actually - an alternative would be the 15l exped cloudburst, but that has no hydration sleeve).
Greg, with my budget, I'm not sure I can get below 20l in total - my bag, bivi, tarp and mat are about 2.5kg and fill one bag; I'm using an old MSR pocket rocket & ti mug for cooking. Although I take your point totally, with food and water I don't think I can do it; especially not for more than one night. I guess a frame bag may be the option but the triangle on an 18" a medium scandal looks pretty small to me.
TINAS - I use a pod sac too, and it is flimsy and short and fat, as you say. I'd like to find a longer/thinner bag too. I don't have a problem with it hitting the tyre with a harness though (100mm scandal, flat stem but not upside down).