Harris Hawk over the Garden

In one of the short really sunny spells yesterday, I was down in the Kitchen making coffee (what else would I be doing!) when through the open window I heard the Lesser Black Backs that frequent the canal making a hell of a racket, so I guessed there was a Raptor about.

So I popped into the garden and saw quite high about 10-15 LBBGs mobbing what I thought was a Buzzard. I popped back in the house and got the camera and took a few snaps of the Buzzard. When looking through the camera, I thought it was an unusual Buzzard, perhaps a juvenile.

A bit later when I was processing my raw images I thought, that's not a Buzzard but I must admit I hadn't a clue, so I popped a cut down photo onto the Bird Forum and asked what people thought, I immediately got back that it was a Harris Hawk! I then did I search and checked my photos against the images of Harris Hawks and yes that's what it was:

Of course this must be an escapee, but where from? I can't see any rings or tags on the photos, but it was very high so not the best pictures.