Zig27
Last night WAS hot, and tonight is probably going to be hotter still, so sadly, your options are rather limited. You are obviously doing the right sort of things, like getting down to a 2-tog duvet and keeping your curtains closed during the day, but you could go a bit further by opening your windows as wide as possible once the sun gets off them, to get some cooler air in.
Reducing the weight of your bedding further will also help. Others have suggested using just the quilt cover on its own, or you could go even further and replace the quilt cover with just a flat cotton sheet, as DH and I have been doing for the past week or so. Admittedly, it feels a bit strange at first after spending much of the past nine months under a duvet, but the end effect is worth it.
Finally that nightie! Lots of posters have recommended not wearing anything in bed, so is it really necessary in this heat? Again, if you haven't tried sleeping in the nude before, it might feel strange for the first few nights, but for many the habit has persisted long after the weather changed. After all, many Mumsnetters admit to sleeping nude all the year round, not just in hot weather, so you would be in good company.