Which divan and mattress

I have a standard double Vi Spring Herald supreme mattress with pocket spring divan base which needs replacing. The mattress is a 'softer springing' style
I'm moving to a smaller apartment and need to replace with a 4' double.
I weigh more now than when I purchased the Vi Spring (92 vs 84kg) but still find the mattress very comfortable.
I have severe osteoarthritis of the spine and have had five surgeries and find firmer mattresses give me back ache as I don't feel the natural curves of my spine are maintained.
I welcome your advice and recommendations.
Many thanks