A female Pied Wagtail Motacilla alba (ssp yarellii) in breeding plumage was by a watery flush in a pasture just inland of the path. A grey mantle, with a dark grey as opposed to shiny black cap, black not connected between nape and throat makes it a female;.a blotchy dark (rather than uniform medium) grey on the back, significant areas of mid-grey - very stylish - (rather than light grey) on the flanks, makes it a female ssp yarellii rather than a female ssp alba. The brown flight feathers suggest this is a first summer bird, retaining the juvenile primaries from last year. The greater coverts have greyish fringes but quite broad white tips, which apparently also fits yarellii. There is little blending of the grey back into the grey-black of the nape as you might expect in a female alba.
More of a hint of a darkish rump in the picture below:
This looks more clearly like a blackish rump. The pictures of this one bird do vary a bit with the light (compare the first picture here) but this looks quite diagnostic.