If you look back in the archives there was some discussion of this several times. Unfortunately I cannot remember what material it was that Budgy said sleeps very cool. Was it silk? Maybe he'll chime in.
I have heard that a wool topper is cool in summer and warm in winter. I have one and it seems to be that way to me. However someone told me that wool always sleeps warmer not cooler. I have not found that to be the case but it's all very subjective. If you read the web sites of those pushing wool, that is what they say: cooler in summer, warmer in winter. Makes sense to me. But who knows? I would say that some kind of topper should do the trick. Personally I think that while Talalay latex is cooler than Dunlop, both are rubber and to me rubber sleeps hotter than most materials except for memory foam. The problem is that memory foam causes you to sink in, and transfers heat from your body into the foam and vice-versa. Memory foam sleeps hotter than anything imho.
Talalay does not seem to sleep hot to me but I think someone with issues with sleeping hot might think otherwise.
So basically I'm a wealth of information none of which is that useful! All I can tell you for sure is to stay away from memory foam and maybe try wool as a topper. And ask Budgy what sleeps cool, I seem to recall him giving some good answers about this some time back.