Replacing old mattress and topper

Hi there, we're looking to replace our old mattress which we've also added a foam topper to. We quite like the softer feel that the topper gives, but I do find it too warm in summer. We're both around 11st or under and are looking for the best option for a new mattress. So far I've looked at the Pocket 1000 and Pocket 1500 (size double). What would you recommend as the best option for us and should we also think about adding a separate topper or not? Our bed has slats for support and so from reading your site I believe we should be looking at adding a sheet of MDF or plywood.