Unhappy Customer

I lay-buyed a bed from The Bedshop in Tygervalley last year. I'm unemployed and in remission so I couldn't afford to purchase it cash. My last and final payment was in December and I was so excited to have the bed before Xmas. Last month I heard the mattress springs making a popping sound. Then I also noticed that the mattress had a dent on the edge. I contacted the store in person and informed them via email. They requested that I forward pics. After they received the pics they informed me that they are unable to furnish me with a replacement because I have marks on the mattress. They advised how to get rid of the marks (water spillage). I will now try to get rid of the marks but is this really such an issue given the fact that the mattress has structural damage? Surely, they need to replace it given that this is still within my guarantee period.