At the moment we're under a duck down duvet and it's too hot, but I think we have a low-tog synthetic duvet somewhere that normally works in Spring.
For summer, I've got a large and extensive collection of patchwork quilts (some antique, some by my mum!) and when it's really hot, we use those with a double sheet underneath..
Nope, we never really moved away from the late 90's Shabby Chic thing here.