Best mattress for weight

Hi
We have a slatted bedstead (not raised, but nailed on), and would like to know the best mattress to choose. We currently have a Hypnos (called Firm Rest 3 by John Lewis) that we bought about 6 years ago. The mattress was fine at first, but now has become too hard for us!We are a couple. I am 5ft 8 and 165 pounds, and my wife is about 5ft 6 and about 160.Can you recommend any mattresses or mattress with bedstead,either double or kingsize. We have limited storage,and currently have storage boxes under our bed (we have a double).Kind regardsMike