In reply to peebles boy:
Make sure whatever you get is sized to go over your other gear (that includes things like ice screws racked on your harness, gear over shoulder etc), has a permanently attached hood (popper ones pull off when you take slings off from over shoulder) and preferably has pockets inside to dry gloves in.
This one looks great value for money when I saw one in the shop
http://www.gooutdoors.co.uk/lowe-alpine-glacier-point-mens-insulated-jacket... or get the Decathlon/Simond
http://www.decathlon.co.uk/warm-mountaineering-jacket-id_8271763.html one for 50 quid.
The Lowe one for example has double the weight of insulation per m2 in the body than the RAB one linked above. Perhaps I'm just soft, but personally I think you might as well have a belay jacket that is proper warm - of course a warmer jacket weighs a bit more, but weight of insulation isn't the main factor in the weight of a belay jacket, so you'll get lots more warmth for not too much more weight if you see what I mean.
Post edited at 18:42