SERTA beds _ so much for back support

I bought a queen size SERTA First Perfect sleeper at Dial-a-Bed. Chose the bed for back support as I have chronic lower back pain. Less than a year the mattress started sagging. Contacted Northgate branch where bed was purchased and spoke to a gentleman that promised the world, phoned SERTA and said that it was under guarantee and that they will replace product. No feedback since and when I followed up today the gentleman is no longer working there, no calls have been logged. I have paid more on physio costs over the last few months than on the bed itself. I cannot go on like this, sleeping on the ground is better than on this bed. The product is definitely not suitable for that that they advertise and sell it for. Please can someone help!!!!