Check out this harrier!

Whilst out doing fen flora quadrats in the North Broads the harrier/s below flew over several times. As far as I can tell they are juvenile Marsh Harriers but why this plumage?? Never seen anything like this before so whether pure MR or hybrid theory who knows. There were at least two individuals showing the white belly which I  also find slightly bizarre.

Any ideas or anyone seen anything similar?