I am interested in the Marks and Spencer's Ortho 1500 king size mattress extra firm. I believe they are made by Reylon. I have read bad reports that they are are a bad mattress, the ones bought at benson for beds. We are both about 12-14 stone and like the firmer feel, the mattress we have now gives us both a bad back. Could you give any feedback on this and recommend a mattress from you. I did see a mattress on your website Origins Pocket 1500 but it says medium. Would this be a comparison ?