Pied and White Wagtails are very variable and in my opinion, White Wagtails are hard to differentiate from Pied, particularly in the Autumn when there is also an abundance of juveniles in partial moult. This gallery illustrates different motacilla alba (yarellii) wagtails.
Pied Wagtail
Pied Wagtail - Motacilla alba yarellii
Pied Wagtail - Motacilla alba yarelliiPied Wagtail - Motacilla alba yarellii