Bad mattresses, bad service

So last year August my husband and i bought a new bed from slewpmaster on account. As the bed starts to fold on where your body lays we complain and recieve a new mattras about a month ago. And now the mattress is worst. So i called this morning the complaimt center so that they can come and take the bed and refund me or sort the bad matress out. And they dont want to assist me as i only have my account number and no proof of purchase or invoice number. But i pay a montly instalment? They cant check any invoice on your sleepmaster account number.? This is bad! Bed not even a year old and its a wast of money.