Have you tried a blizzard bag Ian?
http://www.blizzardsurvival.com/prod...d-survival-bag
I've bivvied in the alps using just one of these without a traditional sleeping bag, and found it to be comfortable enough, even in fairly poor weather conditions.
Admittedly I did have more and heavier weight clothing on than I would take when running.
They are made up of lenticular cells and when opened out fill with air and insulate the body.
They aren't the most robust items, although I've used mine 3 or 4 times now and it's still serviceable. You are right about them not rolling up as small again though.