I got in an hour metal detecting this evening and found this almost immediately.
It is a silver penny of Henry II minted in London by "Davi" between 1180 and 1189. This adds a king I didn't have in my collection. I now have the series of Henry II - John - Henry III - Edward I - Edward II - Edward III.
I now need Richard the Lionheart to fill in the gap. He was the brother of John and reigned after the death of Henry II
It's a great feeling to pick up a coin that nobody has touched for well over 800 years in a world that was so different. Even more so as it was only about 200 yards from my front door.