You've effectively told the retailer that you believe that the mattress was poorly manufactured (i.e. it had an inherent fault when you purchased it). The retailer is saying (through Slumberland) that the problem is related to mis-use of the mattress. You need to argue your case forcefully. Try a formal letter to the retailer, demanding a repair (which seems to be impractical) or replacement (which seems to be a more sensible option). Mention that, if you don't receive an adequate response, you'll not only refer the matter to trading standards but also seek the involvement of both local and national media.
If you still get nowhere you could make a written demand for compensation (because the reailer has failed to meet his statutory duty to remedy the problem), threatening legal action. Then, if necessary, using Money Claim Online to seek redress:
https://www.moneyclaim.gov.uk/csmco2/index.jsp
Chris
PS: Don't bother trying to pursue Slumberland under their (so-called) guarantee. A guarantee does not form part of your statutory rights and Slumberland only state that they'll remedy faults 'at their discretion'. (Which effectively makes the 'guarantee' meaningless anyway.