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These photos confirm that it is a Pied Wagtail Montacilla alba yarellii in my opinion.
It would be unusual for a White Wagtail (M. a. alba) to have such dark flanks, the upperparts do look a shade too dark, and, for a winter bird, the black on the head and breast seems a bit too extensive.
Definitely...far too dark for alba, I agree, from these photos.
My Flickr photos...
Thanks. It never occurred to me to consider white wagtail. I'll look more closely at other wagtails this Winter.
Look for White Wagtails in the spring (Late March- mid/late May), and also in autumn, although they are hard to pick out in the autumn because young Pied are similar. A few may be in the UK during the winter but they are rarely recorded during this season.
Lat/Lng: 51.5875, -0.1186
OS grid ref: TQ304893