In reply to RossWoodruffe:
Not quite the reply you're looking for but, I have used the Mountain Hardwear Thermostatic Hooded jacket as a light belay Jacket for UK winter. It has less insulation than the RAB Nimbus and is a fair bit lighter.
But I think it really depends on the layers underneath and the conditions. I had a long sleeved merino base layer on with the Patagonia Nano Air as a Mid Layer and the RAB Exodus softshell as my outer layer. I generally run quite warm anyway. The conditions were not quite scottish though. Overcast with a light wind and about -5ish from what I remember.
In comparison to the ever popular ME Fitzroy, the Nimbus has more in terms of insulation weight (in theory making it warmer) but does have stitch through baffles (in theory making it colder). Plus it doesn't have the gimmicks associated with a dedicated 'belay jacket'.
Apologies for not actually having used the Nimbus, this is just my 2p worth