Looking at pictures on ST and Northumbrian Birding earlier today of the two 'Blue Headed' Wagtail at Cresswell Pond. One of them has an obviously paler head and at first I began to wonder about the Eastern race beema I even rang ADMC to get him to take a look at the pics.
A bit of digging tonight and I think it more likely that it is an example of what has become known as 'Channel Wagtail' the best description of which that I've found is as follows:
"Nominate Blue-headed Wagtail and Yellow Wagtail form a narrow hybrid zone in northern France. Birds from this zone vary in appearance, but one type, which resembles nominate Blue-headed Wagtail (except that the blue tones to the head are paler and more mauve and the white the head is more extensive, particularly on the throat, ear-coverts, and supercilium) is colloquially referred to as Channel Wagtail"
I'm not sure whether this hybrid has been recorded in Northumberland before will need to do some further digging. I have attached a video from Surfbirds of a similar bird from April 2008 considered to be of this type for contrast.