Thanks for the comments. When I took it I was well-pleased with the focus and exposure, which is not always straightforward with black and white birds, but on later reflection the image does look flat, and it must be the sky. That said, it is easier to take photos underneath what is effectively a huge diffuser, but you can be left wanting. And of course what little detail there was in the sky disappeared when opening up to properly expose the bird. I wondered about somehow employing some PS magic to turn the sky blue, but decided against it. I'm perfectly happy to get rid of a distracting twig or two, but draw the line at changing the entire circumstances.
I also would have hoped for sharper wingtips at the set shutter speed, but I guess I wobbled too much. As for the beak, either the bird is a 1S, or had been recently probing in the mud.
Regards
Jonathan