is this a Grey Wagtail?

is this a Grey Wagtail?

Observation - is this a Grey Wagtail? - UK and Ireland. Description: One of a small group of birds that appeared to me to be behaving in typical wagtail manner - short, fast flights near water. I assumed it was a Grey Wagtail (Motacilla Cinerea), but checking my RSPB Handbook, I see that males have yellow chests and black

One of a small group of birds that appeared to me to be behaving in typical wagtail manner - short, fast flights near water. I assumed it was a Grey Wagtail (Motacilla Cinerea), but checking my RSPB Handbook, I see that males have yellow chests and black throats, females white throats & chests. This bird has a yellow chest and white throat. So - an immature male? or something else?