Is This a White Wagtail or a Juvenile Pied Wagtail?

Is This a White Wagtail or a Juvenile Pied Wagtail?

Observation - Is This a White Wagtail or a Juvenile Pied Wagtail? - UK and Ireland. Description: Grey back? Still not convinced one way or the other. The fence it's perched on is right next to a large lagoon at Rutland Water.

Grey back? Still not convinced one way or the other. The fence it's perched on is right next to a large lagoon at Rutland Water.