Help choosing a mattress please

I'm on the look out to replace our Dreams double 2000 sprung mattress as its about 10yrs old now and I've noticed some lower back pain in the mornings recently.

I am 5'8" 10.5 stone and my husband is about 13st, we both sleep on our sides and are used to a medium firm mattress with a micro fibre topper. I'm thinking we need something supportive with a medium tension but soft feel. The bed is a slightly sprung slatted bed (the type you can lift with storage underneath).

What would you suggest please? Budget is not limitless but I want something that will last us again. Thanks