In reply to 1poundSOCKS:
I agree the Arcteryx Atom LT is great and is what I carry on longer alpine rock routes where I'm carrying a small rucksack or a large bumbag. For shorter UK routes I'd probably go for the windproof-jacket-and-shiver option, e.g. Montane Featherlite.
I got my Arcteryx LT half price; if you don't get it as a bargain it's still good but rather expensive. A Montane Prism is what I had before and is a similar but cheaper alternative, but cut, fit & finish is not in the same league.