Should we accept a replacement mattress if ours is faulty?

I'm 5'7", 8.5 stone ; my wife is 5'2", 10 stone ; we're both in our 70's & fairly good health excepting my wife's mild asthma. August of last year we bought a 5' John Lewis "Special 12000 Angora Mattress" medium tension. The result was a regular back-ache for my wife. The Harrison Spinks rep inspected it & said that it was faulty, we should have been advised to select a firm tension, & intimated that we needn't turn the mattress as often as the literature recommended ( bearing in mind it's weight ). John Lewis were brilliant, said that we could choose another or have a refund. We duly received the same type of mattress with firm tension in April of this year. Sadly my wife has now suffered lower back pain in varying degrees most mornings for the last 3 months. Upon inspection the John Lewis rep found no fault with the mattress & said that we should have been turning it weekly for the first 3 months. I said that in view of the weight ( 57Kg ) & my age/stature I should have been advised differently.
Submitted on 2015/09/30 at 12:09
continued !!
The JL rep said that we could choose another mattress or have a refund, however in a follow up call to their store we were told that the rep had not mentioned refund. If we’re stuck to one of their mattresses & a partial refund I was thinking the Vi Spring Dartmoor in medium tension & a latex topper from yourselves. Would this combo work bearing in mind that I would have to struggle or ignore the turning process. If we manage to get a full refund I’ll call on you regards Latex mattress & topper. I know that you latex mattress is about the same weight but suspect that it would survive less frequent turning. I’d appreciate your advice.
Thanks, Bryan.