In reply to Cadairmanuk:
"packs down well and a comfort level around -5/-10. "
Of course I cant tell from your cold british dampiness,
but still I would recommend to think system, not miracle,
where we are we use from bottom to top up to -10° Celsius:
1.) Therm-a-Rest Original Z Lite or Z Lite Sol
2.) a bag with 500g high quality (fat, no live-plucking)down, bag in total 1-1.2kg
3.) The Pieps MFL Double Alien Double Bivybag
ad1:
often shortend according to body lenght this is the by far warmest/lightest mat to go. If going colder or for several days I also take a short thin Exped Synmat or Downmat with me to protect the body core.
ad2:
cheap live-plucked and quality down make a lot difference, you will recognize after the second cold night in a row.
Also bag design. I did have bags from highly respected brands that either awkwardly distributed down or where not able to keep it in places I need.
If you know your needs regarding down distribution well and especially if want a bag for more humid conditions go and talk to PHD (
http://www.phdesigns.co.uk/ ). I have two bags from them: both very, very good quality at a fair price. They hold/concentrate down in the foot box and around the center, even on back, very well. I've slept in my Minim 400 at -5°C with all and my duvet (Arcteryx Solo) really comfy. Using a down jacket I'd even use the Minim for for -10° to -15° nights in a tent.
If you plan to bivy in snowcaves the Minim is already overkill.
If you know and like the down distribution of Valandre you could also go for the LaFayette (
http://www.valandre.com/eng/Sleeping_Bags_and_Outerwear/sleeping_bags/lafay... ): 564g fat down, 1024g in total, good for comfort up to -12/13°C. The folks at Valandre are also very competent and happy to help.
This is a proven and widely used bag at a fair price (below EUR 400) but many here consider it being overkill, even in winter as you will anyway try to sleep sheltered in a snow cave.
Don't (!) go for a big cheap heavy bag with a full zip which you will use only once. Buy light, high quality which doesn't need venting to prevent you sweating.
ad 3:
Pieps MFL Double Alien Double Bivybag is standard, you can buy cheaper at the Alpine Club here, has good venting capabilities and is highly versatile. Dont even think of membranes. This is to protect from humidity in snowcaves and tents and add a few degrees of warmth.
If you risk unplanned bivy's in winter you carry a blizzard bag (
http://www.blizzardsurvival.com/product.php/100/blizzard-survival-bag ).
hth
PS: PHD and Valandre are capable of repairing/tuning gear inhouse, others,
especially big general brands don't.