Just thought I'd add some extra links for anyone interested in seeing other comparison and discussion on this hybrid wagtail.
Also after some digging and a helpful email from Ian Fisher it appears that 'Channel' Wagtail has been recorded at least twice before in 2006/2007. The 2006 bird at Cresswell Pond on 27th April pictured in BiN 2006 and on Richard Dunn's website and the 2007 bird at Monks House Pool in May.
Ian commented "I think most 'Blue-headed' Wagtails seen in Northumberland are of this type. I have rarely seen a proper Blue-headed i.e. like the one in Newton Stringers blog at Newton in the county, most appear to have the 'powder' blue head etc of the 'Channel' Wag type."
Reviewing images on Birdguides many of the birds seen on the East Coast do indeed appear to show paler blue crown and nape, although not all show the extensive white chin & throat of the Cresswell bird. Ian also noted that "the amount of white on the throat of all these pale headed birds is not too brilliant for what I would call 'pure' flava either"
A couple of links to birdguides images that are useful on this point
White throat & Chin
Pale Blue but lacking extensive White Throat