Submitted this as a deal request yesterday, and then carried on hunting for cheaper, but it turns out this is cheaper than anywhere else :) - for £44.76 you can get:
Super King Size Fitted Sheet
4x Oxford Style Pillow Cases
Super King Size Duvet Cover
in Plain White 400 thread count Egyptian Cotton.
You need to use promotional code DCTV to take 20% off, and then you can knock a further 10% off by going through Quidco too, which brings it down to £40.28 delivered!
You can also purchase these items separately; this isn't a set, and the other sizes are similiarly excellent value.