Ah, in the last couple of days I read somewhere (will try to locate reference!) of the Ossulston, located at the North East of Hyde Park (this would be around Speaker's Corner/Tyburn, I suppose?). Mind you, the extract IIRC was written some time ago, by an antiquarian-type historian l- Stow/Camden/Leland? (Gaaarn I can't remember and it was only 2 days ago! argh) Anyway, it said that the area of Middlesex known in the early medieval period as the Ossulston Hundred was named after this ancient stone. Perhaps a meeting place like Kingston.