Mattress for heavy person?

Wondering if you can help me please. I weigh 27st and have been looking for a new king size mattress. I mainly sleep on my stomach or side. After reading some reviews I was considering buying a latex mattress. I have lower back pain which is definitely being exacerbated by my current spring mattress. I can see that you have a couple of matresses for 'heavy sleepers'. I definitely need a firm matress as I sleep much better, whether spring or foam. Both of the heavy sleeper matresses say that they will feel medium to me. How do they compare firmness-wise with the Fusion 4 range?

Would a Fusion 4 matress cope with 27st?

Are there any spring mattresses in your ranges that would cope with my weight?