dreams are always pushing their half price sales but i didn't find them cheap at all, when you go in there theres only a few beds at half price and the mattresses which is the expensive part arent reduced by much, if at all.
my osteopath did tell me that the best mattress type for your back is a pocket sprung one with a thick layer of memory foam on top, can't rem why.
my be worth having another look at dreams now the jan sales are on and the high street retailers have had such bad xmas sales. you can also try bensons and sleep depot. apart from that have a look at smaller independant bedshops who may have less overheads and better deals.
you could try ebay but i dont like making such an expensive purchase without seeing it and trying it first.