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Grey back + yellow breast = grey wagtail
It's a very nice formula, but this is a Yellow Wagtail! Yellow Wagtails come in a variety of plumages depending on the subspecies, including the one above, which is superficially like a Grey Wagtail. The give away for Grey Wagtail is the very long tail, whereas this bird has a shorter tail in comparison.
My Flickr photos...
Lat/Lng: 55.1787, -1.6165
OS grid ref: NZ245872