In reply to Emily_pipes:
Emily, the Winter Sun is a good jacket, but as a pure belay jacket I think it falls down due to a few issues. There are better pure belay jackets around for the money (IMO only).
Bear in mind these are just my thoughts, and you might see some of them these "negatives" as "positives" (especially point 2):
1.) The Winter Sun now uses 100g Primaloft Eco which is neither as warm nor as hydrophobic as Primaloft One - if you look on the Primaloft website it suggests Primaloft Eco is roughly 2/3rds as warm as Primaloft One.
2.) It also has a Gore Wind Stopper lining which is basically Gore Tex without the taped seams. This is good for reducing wind and keeping some water out, but it means that if the jacket gets wet on the inside (which is VERY likely if your throwing it on over all your other layers in winter) it will not dry out easily from your body heat.
3.) The hood is not very big and hence very tight when worn over a helmet. This also means you can't close the zip fully on some occasions.
Regarding warmth:
There are many jackets on the market which are specifically designed as belay jackets, off the top of my head ME Citadel (200g Primaloft One), ME Fitzroy (100g Primaloft One), Patagonia DAS Parka (170g Primaloft One). Out of these the ME Citadel is the warmest but also the heaviest at 900g.
If I was to buy a belay jacket tomorrow it would probably be the Patagonia DAS Parka beacuse a.) it is very warm b.) it is not overly heavy c.) it dries out easily d.) the hood is excellent, e.) the internal stuff pockets are huge and easy to throw gloves etc into, f.) build quality is high.
Having said all of which there are some very good deals (try Outside) on the Womens Winter Sun at the moment, and it might suit your needs perfectly.