What is it?

I'm relying on you blippers....

I can't figure out what sort of bird this is. It was seen in a Cambridgeshire country park. It bobbed or twitched its tail like a wagtail but I can't ID it. Closest I can get is Grey Wagtail except this bird had a white throat?

The yellow/orange belly and under tail were quite bright - so I thought it would be easy to identify.

HELP!