Bad back, mattress to suit?

I suffer from chronic back pain and usually sleep badly in my current bed (a ten year old king size Relyon Royal Collection cashmere supreme medium with 1360 hand nested pocket springs in cotton calico and three rows of hand stitching, with layers (number unspecified of course) of lambs wool, cotton felt, cashmere and mohair) and find myself aching most mornings. If I sleep in a different bed occasionally, eg on holiday, I often find I am not suffering as usual. The premier inn Hypnos bed was good for one night but I am not confident enough to buy one on that single experience, and anyway would prefer a mattress that can be turned regularly. We did buy a Hypnos topper to see if that would help. It did for a while but the "new bed effect" didn't last very long. Obviously I can't change beds weekly so would love some advice from you as to what you think would suit me best when buying a new mattress. Trying them out in shops for ten minutes is a waste of time. I weigh 11 stone (sometimes a bit less) and my husband (who will sleep on anything!) weighs 16-17 stone. Our base is a solid divan, glued and screwed 'for strength and rigidity'.