Yellow breast, grey on top of head, dark streak through eye.
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From the visible bird the yellow looks a very deep colour and appears to go right up the throat. It reminds me of a flava Yellow Wagtail, but obviously the picture is hardly clear and I am sure you know what you saw! Did you have any more pictures, though?
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I wouldn't generally expect a Grey Wagtail to be quite as bright yellow as this unless it was a male in full breeding plumage (in which case the black throat should also be visible in the photo).
Without other photos (or a more detailed description) I wouldn't be prepared to confirm that this is a wagtail!
this wagtail was seen in flight also and the tail was distinctly long. It followed the water flowing down the ditch whilst wagging its tail vigorously. This photo was taken today 6th March 2012. We could not see any blck bib with naked eye. I agree that the yellow in photo is deep and bright, which surprised us.
Your additional details confirm that it is a wagtail - but is it a bright Grey Wagtail, or an early Yellow Wagtail (perhaps of the Blue-headed race)?
Did you notice anything about the colour of the back?
Lat/Lng: 55.3836, -4.0244
OS grid ref: NS718118