Bad back on waterbed

I'm 10.7 stone, 5 foot 6. I had back pain and was uncomfortable at night and i thought a waterbed would make the pain better having researched it i bought a waterbed with a waveless msttress. This felt too firm so i went back and bought a semi waveless mattress. My back progressively got worse so after 2 years and convinced the bed wss to blame, i cut my losses and got rid of the bed. I've now got a medium firm hovag from ikea on a slatted base, this is too hard so ive added a john lewis topper but alas im still not having a good sleep and i would describe my pain now as chronic lower back pain, pain in hips and pressure point pain whilst seated, and I never wake up feeling refreshed
I've looked at many websites and today I was tempted to order a mattress from eve, casper or zing as I feel desparate for a good night's sleep then I came across your website and hope you can help me as you seem understand people's sleep needs.
I feel I need a soft mattress that I can sink into but also has support. I sleep mostly on my front and side although I try to sleep on my back, my budget is around £500. Can you recommend a mattress