Orthopaedic mattress is dipping but still too firm, please help.

Good Morning. I am having real (and painful) problems finding a suitable mattress for myself and my husband. Please could you help. I am 5'6", weighing in at 8 stone 9. My husband is 6'4", weighing in at 13 stone. We sleep in a Superking sized bed due to his height and preference for room.
Typically, we have been sleeping for the past 3 years on either a Firm, and what has been described as an Orthopaedic Firm mattress (not through necessity - just the description), sat on a slatted base. However, due to financial demands at the time not a huge amount of money passed hands and I suppose you get what you pay for. However, I have found that the mattresses have dipped in the middle (regardless of being firm) and put undue pressure on my upper back and shoulders. This has lead to numbness in this area to the touch and diverted pain down in to my forearms and elbows. It's almost like sleeping in a slight spoon shape, yet still firm not soft. As well as this, the top of the mattress has the usual mounds and divets you see on a springs/" title="pocket spring">pocket spring. This has meant that half of me can be lying in a hole and the other side of me raised up, causing even more issues beyond just comfort. We recently tried an Emma mattress (based on Which reviews - though I wasn't keen on the materials they used), and whilst the flatter top did resolve a little of the issues, the quality and smell of the mattress was disappointing. As you can see, I'm overwhelmed slightly at what I should be buying. I seem to sleep on firm mattresses that sink in the middle but still put pressure on my joints. Confusing! I have set aside a decent amount of money to invest in a bed (mattress and divan) that can offer me and my husband a decent nights sleep! Any expertise you can pass on to me and recommendations would be wonderfully received. Warmest, Julie.