Best mattress for back ache also with hollow back

Hi. I'm ashamed to admit my partner and I have been sleeping on a mattress that is now 20 years old and our backs are suffering terribly.
We need a new bed and advice.
I came across your website over a year ago and got my sister to buy her bed from you off the excellent advice given online. I think she got an origins pocket sprung in the end but i haven't really tired it.
We have an Italian Relaxsan Water Latex mattress on a horrible metal slated frame. It was once so comfy but after i hurt my back doing DIY and my partner hurt his too, I wake in the morning feeling tired and with back ache and wake several times a night with constant aches. As there are no springs though, you can't put it down to a spring in your back or anything.
I weigh 9st 8 at 5ft 7 and he is 12st 3 at 5ft 9.5. He has broad shoulders and in our tiny double bed tends to sleep on his side on firm pillows. I sleep on my front, sort of in a twisted recovery position on a soft pillow but trying to sleep on the recommended back gives no support. I also have a hollow back and can fit my hand fairly easily under my lower spine but not really under my bum or upper back.
Honestly, I have £900 for a bed with potentially £300 more, unless i go more into debt, such as I'm trying to avoid. I'm very wary of trying anything different from foam and am attracted to 100% latex and would like a topper to protect my bed. I also want king size, ideally euro king 160cm. I hate memory foam and like instant rebound. My other half is not bothered. I sort of like the best I can afford.
I was thinking of buying a cheaper bedstead so i can get a better mattress. I really need about 5 inches under the bed to position it over a harth we have in our room. Please can you offer advice? Thanks