Calling blip ornithologists...

...I took this picture for my friend who thought that it was a silver rook or raven, but then investigated and no such bird exists (or so he thinks), his next thought that it was the mix of a raven and a pigeon that got freaky.

He really would like to know what it is (I have to say that I really couldn't care less but he's my mate), I know that there are some very knowledgable blippers out there so if one of you has the answer it would be appreciated (before my mate suggests that its some breed of dolphin!).