Lowena take a trip up to your nearest John Lewis and look for the Four Seasons duvets - these come in one box and are two separate duvets, one for summer use thus a lower tog, and the other for autumn use and a higher tog, then put the two together for the winter and you get the benefit of the much higher tog. they should be light, fluffy and warm. Flat duvets indicate that they are not trapping the warmth.
Like Bodgeitandscarper I have a hot water bottle all year round, thus every single night and this is on my lower back and believe me it does make such a difference to my back - I need this as I have a displaced pelvis and thus suffer a lot of pain in that area and even standing/walking can send my back into spasms and then I cannot move, so hot water bottle it is - marvellous invention.