Mattress bought

We bought a bedroom suite and a mattress but after a month the mattress was sagging in and the springs made a noise. I contacted Corricraft about this but was told that they don't know anything about mattresses and should contact the supplier which is Sleep world. I did and someone was sent to inspect the mattress. Two weeks later it was replaced by a new one. Now after two months we are back to square one and the mattress is doing the exact same thing as the first one. As I have back problems, I get up every morning with backache as the mattress does not support a person's back. <br> The problem I have with this, is that Corricraft sells expensive furniture of good quality but then use a supplier which supplies them with bad quality mattresses. Why does Corricraft as the seller not sort out any problems with the supplier instead of expecting the customer to do so? How can they sell something without knowing anything about the product? I have not contacted the supplier again about the second mattress as I am sure it will be a problem to change it again. Frankly I don't want the same make again and feel we should be reimbursed due to the bad quality of the product.