[quote]As you can see from the Weather forecast for Ben Nevis wind chill is expected to be between -13 and -15 this weekend. For all those that say a down is useless or overkill - At what temperatures would you start wearing one?[/quote]
Wind chill and actual temperature are quite different. Wind chill is all about how cold you feel because of the wind stripping the warmed air close to your skin away, but it's not the same as an actually temperature of -13 or so and a good windproof top will take much of the edge off that.
What you don't really want to do is get down wet, which is why people are suggesting a synthetic like PrimaLoft, which would probably be my choice to be honest. The Rab Microlight down jackets are nice things, but not super warm, the Berghaus Hybrid jacket with a mix of water-resistant down and PrimaLoft would be a better UK option, I think.
Fwiw, I have sat on the summit of the Ben in a down jacket, but it was a freak New Year - 1996 I think - with temperatures literally around -20°C and mostly still air. Of you do get a down jacket, make sure you keep it dry, wet down simply collapses and gives minimal insulation.