Odd Wagtail at Barnes

A morning trip to Barnes on a chilly and grey May day brightened up with a flock of at least 15 Yellow Wagtails on the Grazing Marsh. Initially quite distant they slowly worked their way around the bank. One of them was a very attractive and interesting bird that at first glance I thought might be a Channel Wagtail (a hybrid between flava and flavissima) since the head colour was very pale grey but the grey extended out onto the mantle and scapulars and the supercillium was bright yellow.

I had a look at a few books, mags and websites when I got home and found out that some flava do have yellow supercilliums (called xanthophrys) but I don’t see that a hybrid between a ‘xanthophrys’ flava and flavissima (which might have a yellow supercillium) would also have the pale grey mantle shown by this bird. There is a green tinge to the scaps but it’s mostly grey.

Maybe a hybrid between a Channel Wag and a standard flavissima could show this combination? But I still don’t see where the pale grey mantle comes from. So maybe just an odd Yellow Wag with some kind of colour abnormality? Stunning bird whatever it is. I shall try to do some more homework, but all ideas welcome.

UPDATE - just seen that this has been reported as a Blue headed Wagtail (flava) on Birdguides. I don't see how it can be, but interested in comments.