I've been looking for a double mattress for a while now and finally found what seems to be a great deal. A pocket sprung mattress with 800 springs and damask cover with tufted finish for only £132 delivered! (potentially around £120 if quidco tracks).
Originally £499.99+delivery, now it is reduced to £149.99. You need to add something from the furniture section to have min.£150 in your basket (I added a shoe rack for £4.99). Then apply 20% code FURNDISC (which ends 13/01/2015). Delivery is £8.95 and the total price I paid was £132.93, but if you add something cheaper as a second item, it would be less. There is 5% quidco too, which would make it the total at around £120 delivered :)