Hen Harrier

(Circus cyaneus) See it LARGE
I had to look this one up on the RSPB site - I've never seen one of these birds before and this one is a male.

As it was hovering over it's prey, it was attacked by a pair of kestrels, presumably because it was hunting on their territory.

This is what the RSPB has to say about them:

Of the UK's birds of prey, this is the most intensively persecuted. Once predating free-range fowl, earning its present name, its effect on the number of grouse available to shoot is the cause of modern conflict and threatens its survival in some parts of the UK, particularly on the driven grouse moors of England and Scotland.

In French it is called a Busard Saint Martin