The excellent Greenwich Phantom tells us that the footprint of the old church is shown "by a beech hedge, some of the foundation stones marking out the original position." We can see the beech hedge but we've not spotted the foundation stones. We only read this after our visit so we have resorted to Google Streetview for our photo.
Site: William IV statue (2 memorials)
SE10, King William Walk, St Mary's Gate into Greenwich Park