What Is This bird?

This morning we were entranced by 2 blackbirds fighting in the back garden. We were appalled by their viciousness, as one would pin the other to the ground and peck at it, they locked beaks and then the one on the ground would fight back up and push the other to the floor, flapping like crazy. This went on for quite a while with neither giving any ground. I thought, apparently incorrectly, that in nature there is rarely mortal wounding. 
We were at the point of thinking we should go out and that would scare them to their senses, 
when
whooosh.....
this bird of prey arrived and killed one of the blackbirds, the other flew off quickly.
Sadly they were so involved with their little picture they neglected to look out for danger.

This is a dreadful photo through the window glass and trying to get the whole bird in the picture so we could ID it. 
Still not sure what it is, any ideas?