Odd Yellow Wagtail

While looking for the GS Cuckoo today I saw the above (an early record which I'd post on the Pembs site if I knew how!).
Essentially, the bird looked like a female but was pure white underneath ie. no yellow wash whatsoever not even on the undertail. The mantle was greyish-brown but not grey enough for an eastern-type candidate plus the call was pure, no raspy tones that you usually hear in the eastern clines.
Has anyone else ever seen one looking like this in the spring?
Too far off and too brief a view to get any shot of it unfortunately.