Possible Hybrid White and Pied Wagtails Motacilla alba

possible hybrid wagtail
possible hybrid wagtail
possible hybrid wagtail
These with black on their grey backs may be hybrid White and Pied Wagtails since both species are seen regularly at the UK location, or they may be juveniles turning to adult male pied plumage?
Pied Wagtail female Female Pied Wagtail, Cambridge, UK.
White Wagtail male Male White Wagtail, Cambridge, UK.
White Wagtail male Male White Wagtail, Belgium.
For comparison, above are female Pied and male White Wagtails, the left and centre in the same area.