Mattress for 64 year old 20 stone 6'-1; back sufferer

Hi I'm looking for a new king size Mattress for a slatted bed, I suffer with low back pain. My wife is around 12 stone and diabetic and gets very hot.
We use a 50 mm memory foam topper at the moment but looking for something better. Would consider Tempur but reports all very confusing.
Any advise much appreciated and if possible write up and pictures of any recommendations along with prices.
Many thanks and regards.
Eric Butler