My daughter finds that down irritates her so she gets hollofill (sp?) polyester duvets. She usually gets a double one - a 4.5 tog + a 9 tog which button together to give a 13.5 tog duvet for winter and separate them for the other seasons.

Polyester filling is almost as light as down but it's less expensive though it's more bulky and does not fall round you so well. However it is machine washable, though you need a large machine. We take ours to a launderette nearby which has both extra large washing machines and dry cleaning machines.