Don't know how these compare to other mattresses but thought i'd post as they seem like quite a good deal. M & S have reduced their 'Ready To Go' mattresses as follows:
Single - £79 | Double - £99 | King - £119 - Use Discount code: PCAHHZT for free delivery.
Description: Contains revolutionary foam which reaches its full comfort level after just one hour and provides high levels of support for a good night's sleep
Mattress thickness: 15cm | Single: W90 x L190cm | Double: W135 x L190cm | 5ft: W150 x L200cm