A few years ago I posted a tip on Answerbank about just the same thing. Conservatory that was too hot in the summer and freezing in the winter.
While in my local DIY store, I came across rolls of insulating polystyrene. Its supposed to be for walls.
I "papered" the conservatory ceiling with it, using a strong wallpaper paste mix.
I now have a south facing conservatory that is about 25 degrees cooler in the summer, and is also much warmer in the winter!
I also painted it white and friends and family have commented on the fact that it just looks like an ordinary ceiling.
Its a much better product now than it was years ago, and is also fire retardant.
It has now been up 8 years and is still going strong!
Hope that helps!