Help with ID (description) please

[agrumpycow]

My sister's just sent me an email wanting to know the ID of a bird she's just seen and I can't be certain. She says:
a bird which hovers in mid air 'like a humming bird', looks just like a pied wagtail but with a bright yellow belly & seen near a canal (in Berkshire)
I'm thinking possibly grey wagtail but I'm not sure?

[galanthus]

I'd go with grey wagtail too, more likely than a yellow methinks.

[Pete W]

Your Sister's description does match the description of a Grey Wagtail as described in my books....

[agrumpycow]

Thanks both of you!