Sadly I didn't have my 400mm lens with me on the day I spotted a Grey wagtail in the mud on the other side of a small lake. But this picture is good enough to identify it well. It's a good opportunity to point out some things about Wagtails in general.
You've no doubt seen the monochrome Pied wagtail doing its dipping and dashing, commonly seen in car parks. At the other end of the scale in colour as well as rarity is the Yellow wagtail, which is like our grey friend above but more yellow throughout. And that's the trick - to realise that Grey wagtails are in fact yellow, at least on the underside, but grey on top.