In reply to David Staples:
i asked the same question last year when i was looking for a new bag for scottish winter bivvying.
I ended up buying the rab andes 800, an absolute beast of a bag!
yes its heavier that your usual scottish winer bag and it carrys a harsh price tag but being filled with quality down means that it packs down tiny, and inflates to be massive! i've used it from freezing nights in storms in scotland, all the way to sleeping at 4000m in the alps and never once have i regretted spending the money an not once have i been cold. theyalso use a pertex outershell which is meant to only be for fighting of a slight wind or accidental spills from your flask before you need a bivvy bag. ive found that this is alot tougher than its made out to be and youll get away without a bivvy bag against almost anything but heavy rain.
when it comes to sleeping bags, rab is the way to go. when it comes to warmth, the only thing better than the andes bag, is your bed at home with the thermostat up!
Post edited at 20:30