Is the Simba Hybrid Pro suitable for a heavier couple, around 17 and 14 stone?

Yes, and it is the Simba we would pick for you. The Pro has two layers of microsprings over a steel SupportCore base, so it carries weight better than the standard Hybrid and keeps its shape at the edges when one of you sits or sleeps near the side. At a combined weight of around 31 stone on a double you will get some roll-together on any mattress, so a king (150cm) is worth the extra if the room allows. The Hybrid Luxe and Ultra are softer on the surface and not better for your weights; the Pro's medium-firm feel is the right one.