Found this deal instore today - packs of sizes 5+ or 6 half price. Just checked and it's online too, and says the offer's on from 28/12/2009 until 12/02/2010.
Couldn't find it posted and this is a mega saving if your littlun wears these bigger nappies. For the size 5+, you get 44 in a pack so this makes it only 10p per nappy!
Bought 5 packs at that price - hope he doesn't have a growth spurt now!
Hope this helps some others