Mattress for 20 st individual who suffers from back and hip pain?

Im looking to buy a mattress for my bed . The size would be king size . Im 20st and suffer from back and hip pains which is worsened by my mattress , which at the moment is a Tempur cloud . Im sleeping in a chair as this doesnt give me back ache so im assuming its my mattress causing my hips and back to ache .