Really depends on how much you want to spend!
If the bank account is bursting at the seams, and you want down, try phd, zpacks, cumulus, or maybe even the rab neutrino range. They all do bags that will get comfortably down to low single figures and pack ridiculously small.
However, if you're on the same budget as the rest of us, my suggestions would be:
synthetic - snugpak traveller range - around £35-40 and quoted pack size of 12x14cm (although I have a softie 3, which is supposed to go to 16x16, and that needs an elephant's weight on it to get to about 22 x 17)
down - the vango venom range. The whole range was updated this year, so the 2011 stock is being sold off in various places, including fleabay, at pretty good prices. The 150 would probably fit your needs - and it's down to £47 at a couple of spots on ebay. I've just ordered a 300 from outdoor direct at £72 all-in.
All of the above bags come in comfortably under a kilo, too.
Hope this helps