Thanks for response. I turned off mains, removed key, and figured out that a quarter turn of the key, presumably in either direction, stops / starts water flow. However, the teeth on the end of the key have broken off, so it is slipping without turning. I used a piece of wood with the same profile as an improvised key, but find that it still won't turn. I think it may be jammed, which may be why the teeth broke in the first place. So, no change. The key is back in place, functioning as a plug.

ffry, April 2007

You have obviously broken off the lug on the key and the valve is locked in the open position.
You will have to replace the fill assembly on the jig.

bigclive, April 2007