White or Pied Wagtail?

I took the series of photos below of what I presumed was a White Wagtail (alba) on Wanstead Flats over the weekend. This presumption was mainly based on the mantle colour being rather plain and ash-grey in colour (this was even more noticeable in the field) but given the amount of grey around the breast and to a lesser extent the flanks, I'm now swaying towards a female Pied Wagtail (yarrellii), but I've also seen White Wagtails with a light-grey wash around the flanks, so I'm still not 100% convinced.

The white wing bar also appears to be quite broad and the grey mantle doesn't encroach into the rear crown (what you can see of it) as much as you would expect in a female White Wagtail, so maybe it's just a female Pied Wagtail with a light grey mantle after all? It's just a shame that none of the photos clearly show the rump/upper tail coverts, as this could of clinched it either way (ash-grey for White Wagtail, dark grey for Pied Wagtail).

As always, any thoughts and comments are welcome...