Shakespeare mattress too hot!

We purchased a king size mattress 18 months ago and we both really dislike it. It's a Shakespear with natural latex memory foam. We were initially so hot when sleeping on it that we have now turned it over to sleep on the underside (i.e. the side you are not meant to sleep on) to try and get a cooler night's sleep. The other problem is that it is so heavy and the base is slightly recessed, that the bed is impossible to make - you just can't pull the sheets straight across the bed and tuck them under the mattress. So it is made even more uncomforable because the sheets are often rucked up. The base is slated wood. We have decided to cut our losses and ditch this mattress. So, what we want is a king size (2000mm by 1530mm) mattress, that is firm, cool and not to heavy so that it is easy to lift to tuck the sheets under when making. I slept on a friends guest bed that was firm, had a natural fibre cover and had holes in the sides (presumably for ventilation) - it was the best night sleep I have had in 18 months. The label said 'Harrow - Heriot, open type spring, construction modified foam & fire retardant cotton felt'. Heals said they don't do this make anymore and they also said that what I described was not suitable for anything other than occasional use as it would wear out in 2 to 3 years if used constantly. We would be happy to pay up to £500 for a mattress and a decent night's sleep, can you suggest anything?

I am 9.5 stone and my partner is 13 stone. Your site is really useful by the way.