In reply to Stanners:
Rab Xenon X is perfect for that. Primaloft filled and windproof but thin and light at around 340g.
And what do you know? I have one to sell. I bought it in medium and have worn it on 3 walks. I find it warmer than it's weight and appearance would suggest but just a tiny bit tighter than I like so I've just bought another in Large. I'd say the one I want to sell is a neat 40" chest. It's very dark blue and altogether unmarked - can hardly tell from new, if at all.
If it's your size let me know and we can agree a price I'm sure. Cheers. Martin.