In reply to martinturner:
Have you considered the PHD range?
http://www.phdesigns.co.uk/down-sleeping-bags
They are expensive , but when you weigh up what you get for the money, and treat it as a long term investment, I think they are worth splashing out on if you can afford it.
I've had my Minimus now for over 10 years and in spite of heavy use it's still as good as new, and definitely does what it says on the tin!
Advantages: Made to your choice re zip or no zip, zip length, zip on left or right side if you opt for one.
Very light without loss of warmth, packs down to a very small size in your rucksack, yet regains it's loft quickly when you unroll it. Optional shower proof treatment to the outside.