Many thanks to everyone who offered help yesterday, its very much appreciated and its one of the things I love about flickr. I removed the picture after I found the cure, which is basically that the Long Exposure Noise Reduction has to be switched on. I never needed to use it on my Mark II, so its a bit disappointing, but at least its not a fault. It was definitely a fault on the first copy of the Mark III I had, it was noisy on very long exposures with the NR switched on or off.
I haven't had much time with the new camera yet, this was a quick snap in the garden... I'm so lucky to have nuthatches so close! This is one of the youngsters from this year's brood :) A tighter crop than I would have liked, but there were some ugly things I couldn't clone out.
It looks sharper on black, flickr's playing tricks again!