Grey Wagtail

According to the RSPB:
The grey wagtail is more colourful than its name suggests with slate grey upper parts and distinctive lemon yellow under-tail. Its tail is noticeably longer than those of pied and yellow wagtails.
Very helpful if you happen to have a pied or yellow wagtail's tail to measure against, but I think the white flash around the eyes make it a grey not yellow also the yellow is a summer visitor to these parts.
The birds are found over most of the UK (with the exception of the Northern and western isles of Scotland). They like fast flowing rivers in summer their greatest densities are in the hills of England, Scotland and Wales. In winter they can be seen around farmyards and lowland streams, even in city centres. So I guess we have visitor.
It was perched on the bridge by the City Mill. I caught it out of the corner of my eye and had to look twice; eventually my brain registered and the turn your camera on and move slowly voice kick started me into moving. Although it didn't stop bobbing around I did manage to get a few reasonable shot of it.