Tried posting all the links but won't let me. I have pages of complaints that might help you and two Guardian articles. Can't post email addresses on here for sure so not sure how I can get them to you. But happy to help where I can

I am really sorry you are having these problems. Bensons for Beds know there is a serious problem with these airflow mattresses which are solely provided by Bensons for Beds, If you do a google search you will find hundreds of online complaints about this range of sagging mattresses. I cut and pasted all of them and submitted them to Bensons for beds, who declined to comment but they did give me a voucher for the full cost of a replacment mattress. Needlesstosay I went with a different make Sealy and have been very happy thus far.I approached bensons for beds, contacted Watchdog and the guardian newspaper. I was not going to stop til I got it sorted as £500 is a lot of money. My final action would have been to go to the small claims court. I advise you to go back to Bensons for beds armed with a list of all the complaints online and two guardian newspaper articles which you can find by putting in guardian and bensons for beds into search engine. Let them know you plan to take them to the small claims court to resolve. Good luck.

Would be interested to hear how the new mattress is getting along. I too bought the same mattress (Sleepmaster Airflow memory mattress 5000 Double) back in March 2012 and it has been a nightmare. Have to climb into a sagging hollow in the middle of the bed each night. Have just written to Bensons for Beds and have sent some photos. Can't wait to see the back of this mattress and would never have the same again. Will push for a replacement of a different type or a refund. If neither are offered will head for the small claims court. £500 is no small sum of money for such an appalling mattress.