I've spotted some great prices on Memory Foam mattresses @ Argos. The one I'm linking to is £209.99, and is the Layezee Posturezone 4ft 6in Memory Foam Double Mattress. This seems like a great price for this item. I have a memory foam mattress myself which is absolutely amazing- once you have one, I don't think you can ever go back to the sprung mattresses.
I'll post the links to two more in the forum thread as well, both for the Sealys, double and king size.
Details: This Layezee Posturezone mattress features a slow recovery memory foam, developed using space-age technology. * Coupled with a greater concentration of open coil springs in the central third of the mattress to give greater support where your body needs it most, the memory foam provides an additional cradle of support along the length of the mattress making for a fuller, more restful sleep with fewer waking periods * Modern micro-quilted mattress cover contains generous layers of filling with the deep layer of visco-elastic memory foam incorporated into the top layer of the filling. * Unlike a conventional mattress, memory foam has much fewer pressure points on the resting body providing evenly spread, unrivalled comfort. * The soft knitted mattress top layer allows the memory foam cradle to work to an optimum level and gives a medium firm level of support. * This 13.5 gauge easy care (no need to turn over) mattress will benefit from being turned top to tail every 3 months and will help prolong the life of the mattress.