In reply to TobyA:
To be honest, a combination of conditions.
5 people in a snowhole on WMF (military winter skills) - I had my Rab Ascent 500 in a gore-tex bivi bag (hooded issue, rather than zip up). Went to sleep toasty and warm.
During the course of the night, I woke up and the air felt very damp, but couldn't really work out what was going on - I presumed condensation from exhalation of 5 people, or even some "melting" of the snow, caused by five people. What had actually happened was that the wind had changed direction and very fine spindrift was blowing in the entrance (despite being reasonably well covered by bags etc.) - by the time we woke up, we had a good 6 inches of fresh powder inside the hole, with the entrance being mostly covered. I say awoke - I hadn't slept much. I think the combination of condensation from my breath and fine spindrift had split between and my body heat melted it, causing a reasonable amount of wetness inside the bag, resulting in clumpy bag in one night - fortunately, we were only out for one night and the drying room back at the centre was ACE - lying it across the racks for a couple of hours made it as good as new (although I was worrying all the way back!).
It was this experience that has caused me to doubt the use of down and revert back to synthetic, despite the space / weight saving - I can deal with that if it results in a reasonable nights' sleep.