OCUK for one of the few times I've seen weren't bad at all on prices of the 5850/5870 on launch, I was fully expecting the usual mark up with a few non specialist stores doing normal prices, but OCUk were fine, and for some weeks.
It's hard to know without seeing their orders with distro's directly where the price increases have occured to be honest, but someone's getting screwed somewhere.
Yields don't directly worsen prices in that sense, the yield per waifer didn't decrease, the factories ability to produce waifers went down. But if AMD had 5000 waifers made, or 1000 waifers, the price per waifer is the same to them. It's more likely OEM's charge distro's a little more, distro's charge people like OCUK more and OCUK charge us a little more also, but seeing as companies don't charge a fixed price markup on parts, but a percentage, when the price goes up at every stage, the normal mark up percentage, whatever it is, increases anyway.
When stock eases we should see initial prices again so £190-200 for a 5850, probably a decent drop in 5870 prices due to the 5970 being too good a deal at £415ish which is where it should be now. THe 5870 is a hard enough sell in terms of value over a 5850, when you can get a 5970 for 1/3 more than a 5870 it becomes entirely pointless, I can see the 5870 being pushed down closer to £250 before end of January.