A Black Wagtail

On Monday, just after mid-day, I saw a strange wagtail beside one of the wooded lakes near Farnborough North station.  It was definitely a wagtail: it was the right size and shape, and it behaved like one bobbing its tail up and down as it perched on a branch.  However, its head, neck and upper part of its chest were all black.  Its back was grey and the rest of its body was mostly black with whitish streaks here and there.  While I was watching, it repeated the call "chi-chee" several times and then it flew off into the woods.  I suppose it could have been an unusually dark pied wagtail, maybe a juvenile as these tend to have less well developed white patches than the adults.