Blue-headed Wagtail at Prior's Fen GP
© Kevin Du Rose
This bird is a bit different to our usual Yellow Wagtails that breed in the area (subspecies flavissima). Its very pale head looks like the eastern European Blue-headed race, beema (known as Sykes's Wagtail), though it could perhaps be a flavissima/flava hybrid.
Yellow Wagtail races merge into each other so there may be no definitive answer!