Restonic Gold matress sagging in 3 months!

i bought a restonic gold king size bed from dail a bed in may 2015. paid the full amount. 3 months later matress is sagging. went to shop and saleslady tried to sell me another bed. promised she will tell manager and phone me back. 12 months passed and no call. went back to shop and spoke to manager who promised they will sort it out.. 2 more months and still no reply. sent email to restonic head office and soeone phoned me in november 2015. told me he is just waiting for stock and that factories r closing soon. went to shop again in beginning of jan 2016. manager Primrose promised it will be resolved and that someone will come and look at bed.emailed resotinc again and again no replies! bed is 9 months old and already matress is sagging! what about my ****** guarantee?