In reply to Richardhobbs:
It packs about as small as you would expect the warmest synthetic belay jacket around to pack! It has Primaloft Synergy in it (now known as 'Silver Hi Loft' or something), which is the continuous filament stuff they stick in sleeping bags. It lofts higher, and lasts a bit longer, but no it doesn't pack down small.
What are you using it for? It's hugely warm and unless you're in mountain rescue, going on an expedition or spend a lot of time belaying slow leaders on the coldest days of the year I'd say it's way too much for UK use.