In reply to Scott Anderson:
Have you seen the Summer Lightning range from PHD? The Hispar 300 (rated for -5) is £399 (will be slightly more if you need an XL version). Only weights 580g and looks pretty amazing for summer alpine.
http://www.phdesigns.co.uk/hispar-300-down-sleeping-bag?osCsid=k6j4ld44mkpq...
Alternatively you could look at Criterion (formerly Cumulus). The Quantum 350 (-6/695g/£270) or 450 (-10/805g/£297) might fit the bill.
http://bac-e.com/sleeping/down-filled/criterion-quantum.html
I have a Cumulus bag from before they changed their name, the Prime 700 which has served me well for Scottish winter camping but I'm also in the market for something lighter for the Alps.
Edit: I haven't used PHD bags but they are very highly regarded.
Post edited at 18:29