Grey wagtail nesting in mansion block in central London

I live in a flat in a mansion block in central London, and have plant boxes hanging on the rail of a walkway that joins two flats.  A grey wagtail has made her nest in one of my plant boxes and hatched several chicks.  Is this a rare occurence in London?

I've never seen grey wagtails around here before, and certainly never had one make a nest under my geranium!