New mattress for 'Vi Spring' divan base. 9.5 st 5' 8" i n height

My vi spring mattress is past it's sell by date. I have a super king size bed. I am getting a sore back each night and I certainly can't get my hand underneath my back. I just don't know where to start. I feel I want a natural fibre springs/" title="pocket spring">pocket spring mattress and am prepared to spend up to £3,000. I would prefer a mattress that doesn't need to be turned often - if at all. I have a mobility problem caused by arthritis.