Thanks Martin, I was being a dufus! and was surprised not to see it from Klattermusen.
I think I mistook the '800' for a fill power value. I have to confess 800gm doesn't compute as 'lightweight' to me, but I guess if it is most extraodinarily warm then gms per 'warmth value' might be good and everything's a trade-off.
Still, 300 gm down fill in 500gm worth of jacket to add up to the 800gm doesn't look anything spectacular in the 'lightweight' stakes, though I'm sure it is a very nice, warm, high quaity jacket.