White rumped buzzard?

Essex, today, sunset.

I was watching a buzzard fly past when I noticed it had a white rump making it a possible hen harrier candidate so I took some pictures. Sorry for the quality - I blame the camera - and there was little light and I didn't have time to check the shutter speed.

I've seen at most one hen harrier so I don't really know what they look like in the field but until I saw the rump I would have had no hesitation calling this a buzzard. Its behaviour wasn't quite buzzard though. It was flying lower than I'd expect from a buzzard, it flew past me rather than flying away from me, it twice dived at things on the ground and it generally did more turning and jinking than the usual buzzard.

I know that buzzards are very variable but the consensus on the web seems to be that that doesn't stretch  to white rumps. Any ideas?