Marsh Harrier?

As I was leaving Middleton Lakes the other day I spotted this bird flying in my direction and managed to reel off a few shots (albeit fairly distant even with a 400mm lens!).

I've convinced myself that it's a Marsh Harrier (as there's known to be one pair at Middleton). But now I've had a look at my Handbook, I'm I'm uncertain because it looks fairly similar to a Buzzard.

The underwing pattern and colouring makes me hope it's a Marsh Harrier (because that will be the first time I've ever seen one!)