[Apologies for the poor quality - it flew on top of a building and I only had an old cameraphone to hand]
Overall rather a pale yellow, dipping motion in flight, the beak and possibly the front of face seemed to be black possibly with a grey tail. It was singing rather lustily once it settled, but a very varied song that didn't seem to repeat. It was larger than a wagtail but smaller than a blackbird.
It was in the Eastbourne Station/Enterprise Centre area.
I have lived in the area most of my life and never seen a bird like this.