Time to open up that can of worms known as the pied wagtail. Took this photo today without thinking much of it, then at home noticed very pale back, with brown feather edges. And then the black bib around the neck was different to my other pied wagtail pix. So I wondered if it was a juvenile or a female. But it looked too big for a juvenile, and I have come to asscoiate a yellow-ish face wth a juvenile. So that leaves white or female. RSPB Bird Identifier doesn't distinguish between male and female for colouration, so if push came to shove, I'd go for white.
I shall probably be completely wrong. :)
http://www.flickr.com/photos/steve_dewey/sets/72157623533703702/ (there's birds there - and some are actually sharp!)
I could be wrong but i'll go for Pied Wagtail
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I don't think it's a juvenile either. The ones I've seen recently are far less well marked than your bird as you can see from this pic.
White Wagtails have a very plain grey back. Yours looks fairly mottled with hints of brown. My opinion, for what it's worth, is that it's a worn female Pied.
I would be happy to concede that it's not from someone more expert than me.
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A bird does not sing because it has an answer. It sings because it has a song. (Chinese proverb)