Thoughts on this one

Up Roils Head yesterday in heavy rain and photographed this wagtail. It looked very pale underneath which is what drew my attention. The rain was so bad I couldnt use my bins so relide on the camera to get some record shots. I returned home and phoned dave that I had had a possible White Wagtail. When I saw the blurred flight shot though I started to doubt myself (does it have a dark rump or is it the fact that the camera shake as blurred the image). I seem to remember reading somewhere about varible rump colour on White Wags but can not recall where, any other thoughts appreciated.