how to remove key after refilling worcester 245Bi boiler?

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.