OK, guys,
Now I'm excited! That little "washer" turns out to be what's called a Verval Hawking Ring. This would have been fitted to a hawk's leg by jesses, and (as you can see) would have borne the owner's details. These silver vervals have been dated from the Mediaeval period up to the 17th Century. WHOOPEEEEEEEEE!
Bobbing Court is about three miles from the field I dug it up in so it's possible (only conjecture here!) that the hawk was released, maybe chased after a prey, and ended up dying in this field for whatever reason.
So, another detecting mystery cleared up. Thanks for your interest and comments, guys!
Bob.
PS. Don't we have a member who is interested in birds of prey on our site?