At 11-12 stone we would match you with a Medium spring. The spring is solely determined by your body weight as you need that to provide the support from inside the mattress.
We would need more information in regards to whether you’re looking for a traditional pocket sprung mattress or something different. We also need to know how you prefer the mattress to feel as comfort and support is very personal from one user to another.
Our pocket sprung mattresses can go on to a slatted base as long as the gaps are no bigger than 3″. If they are bigger then we advise to place some sheets of MDF or ply wood over them.
Please come back to us with some more information and we can better advise you from there.