Hi John,
This bird is a Yellow Wagtail. Note, among other features, the prominent whitish fringes and tips to the median and greater coverts and tertials, yellow limited to the vent, far greyer upperparts than any Willow Warbler would ever have, likewise a stronger supercilium, blacker legs than on any Phylloscopus and a much longer hind claw.
Technically, I suppose I should call it a yellow wagtail species, as, while the presence of a yellow vent on such an otherwise grey and white bird takes this outside the typical 'search image' for identifiable vagrant Eastern Yellow Wagtail, I don't know if that species, admittedly far rarer in western Europe than Western Yellow Wagtail, can be completely excluded, especially as that species tends to have a longer hind claw than on Western. However, I'm not saying that it is the rarer alternative, recordings of the bird's calls and/or DNA would probably be required to establish such an identification safely. It's certainly not a warbler, though!
Regards,
Harry