In reply to jacktomlinson:
Another vote for the Paramo belay jacket, though I don't own one myself (too many other jackets to justify another one).
I like the toasty warmth, weather proofness, and I think a smock is less faff to put on whilst on a stance.
As for me, I already have 3 jackets I could use as belay jackets, and has been said, there is a trade off between bulk/weight and warmth.
My old Montane Solo Primaloft jacket is dead light, and adds some warmth, but not as much as my (bulkier and heavier) Mountain Hardwear Compressor, which is itself not as warm as my mahoosive and old Berghaus jacket.
I reckon you try what works for you.
My Berghaus is over 20 years old, the Montane Solo is over a decade old, and the MH Compressor is 4 years old, and they all still work well, and they all get good service on the hill.
You can usually get a synthetic jacket on sale at half price somewhere sometime, or you can look in TKMaxx.
Or you can leave the Big Brands behind. Decathlon do a belay jacket for under £50. Sports Direct will sell you a padded jacket you could use as a belay jacket for £20.
That way you can see what weight and warmth you need before spending £200 on a PHD Zeta...