Buying a new mattress. Your opinions, please.

I need to replace my mattress and am bewildered by the choice, so I was hoping to get people's thoughts and experiences.

I want a king-size mattress for a divan bed. I currently have a fairly expensive memory foam one, which is 8-10 years old. I love the memory foam in that I don't wake up aching or with sore spots. On the other hand, I find it's prone to making me too hot and over time the top has acquired an imprint of where we sleep, with a hump in the middle. The side has also sagged outwards and we find it so heavy that it's very hard to rotate it regularly, as recommended.

Therefore, what to replace it with? I see newer memory foam mattresses that claim to be 'Cool top' and similar phrases. Does this actually work? Is the foam supposed to acquire an imprint of one's body? My daughter says it is, but I don't know!

There are many other types of mattress also available, ones with pillow-tops, ones with wool or cotton or alpaca fillings. One review I read of a wool-filled mattress said it smelt as though you were living in a farmyard, which doesn't sound very appealing!

Then there's price. Obviously one can buy a mattress for occasional use quite cheaply, but is it the case that you get what you pay for? Is a £1000 mattress that much better than a £500 one? Is a £5000 one five times 'better' than a £1000 version? Don't worry, I won't be paying £5000!