I asked around about the stone but no-one can remember it. Maybe my mind is playing tricks on me!!
What I have come up with however, is that an American Aircraft came down in Endcliffe park during WW2. There is a memorial stone there and each year a service is held in remembrance of the crew members who died. Apparently it was held recently.
Maybe you got the park mixed up?
I wanted to change the world but couldn't find a babysitter!