Two single mattresses joined together rather than one double mattress works better for Mrs JtB and myself.
I have had what you call a knackered back since 1966. A lifting accident at work before the days of being taught how to lift.
I was told to get a firm mattress which I did......it lasted a month before going to the spare room.
I then went to an independent store and talked it through with the owner. Tried the one he recommended......perfect.
My back still 'goes' and did so 2 weeks ago but I now get a comfortable sleep.
So.........my advice, go independent as they are not paid commission and can give you good advice.