Sleep problems from expensive pocket sprung mattress

We have a Vi-Sprung Herald Super-King mattress on a slatted base which is medium on my side and soft on my wife's. It was fine for about five years but over the past year I have been having difficulty breathing when asleep - which I initially thought was apnoea. However, I realised it didn't happen in hotels on holiday and must be due to our bed (having also tried many different pillows).
I'm a side sleeper just under 13st and I think I am sinking too far into the mattress at the chest/midriff, leaving my neck bent and my knees too high. It seems the springs have become too soft after just a few years, yet the upholstery is firm and unforgiving. I would like to achieve the opposite effect - very firm support but a soft (but shallow-ish) comfort layer to ease hip pain.
Firming up the bed base (switching from sprung slats to solid, and then topping with MDF boards) gave some improvement as did sleeping with a folded blanket under my chest. Finally I set up in the spare room on a boarded base with a £40 4 inch-deep dense foam block on top - and actually had the best few night's sleep at home in ages. I can breathe freely- although it's hard on the hips! I also really like the smoothness of the foam mattress compared with the heavily indented style of the Vi-Sprung.
Clearly we need a new mattress, but I am extremely wary since this one cost a great deal and comes from a top manufacturer but has proved a poor choice.
I am increasingly thinking latex might be the answer, but would appreciate your advice.