Motacilla alba yarrellii
On flood water in a roadside field.
The Pied Wagtail in the UK is a subspecies of the continental White Wagtail which also turns up in the UK in the spring during migration pretty much every year.
When it walked out of the grass onto the mud I thought I was finally going to get a clear shot of this species but it flew off before reaching the top of the mud edge. You can’t win them all !