This bird is currently holding territory at RSPB Saltholme.
This thread http://www.birdforum.net/showthread.php?t=169438 has some discussion from last year about another grey backed bird on the reserve in 2010 and 2009. This bird looks more similar to the 2009 bird than the brighter 2010 bird. All three could be picked and seperated from the other wagtails up by their call and I have managed to video this years bird so you can hear it for yourselves.
Opinions, thoughts and everything in between welcomed.