Thanks everyone for your input.
I use Sea to Summit dry bags for sleeping gear, so wasn't too worried about it from that point of view.
We use a double wall tent so hopefully not too much condensation related wetting.
More worried about moisture building up over a number of nights in a potentially humid environment. Perhaps, as Son of a Beach and Wayno have said, this
might only be a concern on a longer walk than the OT.
I have decided to use either:
a synthetic quilt from Mountain Laurel Designs rated to 28 degrees F, weighing 700 grams, or
one of two down quilts from Katabatic rated to 30 degrees F (565 grams) or one rated 22 degrees F (690 grams)
I have been very hesitant to use down on the OT, here are some comments from Ryan Jordan, founder of the BBL website, in reply to an email I sent him a while back:
"The Overland can get pretty humid in sustained rain."
"When the weather is good (high pressure, thus - low humidity), regardless of where you are, down is great.
When the weather is consistently rainy *and* cold (less than 40 degrees F), then synthetics usually outperform down, but the level of outperformance is usually only realized when you are out for about 3 or more nights in these sustained conditions.
The worst scenario for down is the 25-35 degree night, when it's raining or snowing. Then, down is a ticking time bomb."
You good people have given me some reassurance though, and down is back in consideration.
If I could ask another question; what do you folks think of the temp ratings of these bags for the OT in Jan? We will also be wearing a beanie and jacket (Patagonia Micro or Nano puff) to bed, and have very warm sleeping mats.
Thanks