Single Bed for Sciatica and Back Pain Sufferer

Hi,I would appreciate your advice for a replacement mattress.I am 80 years old, 6' tall, weigh 77kg, and am still reasonably active. I suffer frequent bouts of Sciatica in my lower back and left hip. back pain history goes back over 50 years, therefore I have in the past always been advised to go for an "orthopaedic" bed. My sleeping habit is about half of the time flat on my back, the other half curled up on my left hand side (turning on my side is painful and usually wakes me up).Two years ago I purchased a 3' single size Sleepeezee Backcare Superior 1000 Pocket Pillow Top Mattress, advertised as medium to firm feel, and a solid top divan base.The mattress is now simply too hard for me, and the foam rubber top ("the sumptious pillow-top finish") causes me to perspire. In other words: I am unhappy with it.I would like to replace just the mattress with a pocket sprung medium-feel one, well ventilated, with a cotton cover, and no rubber or latex component anywhere near it. Naturally I would require fitted handles for turning (some makers save pennies by omitting them!). My budget is around £500.Can you possibly help me to a good nights sleep ??