Recommendation for a mattress topper that's soft but not too warm?

[TheInimitableMrsFanshawe]

After sleeping on a bed with a mattress topper on holiday, I think it's what's needed to improve the feel of our mattress. We paid a small fortune for the mattress and it's only 6 years old so I'm not keen to replace it just yet.

Any recommendations?

We have a bamboo cover on our memory foam mattress topper - it's ssssssoooooooo soft!!!