Hello Magpie.
Yes your are right. It is a John Tann Grille lock, as you probably saw the lever springing is steel with the Tann style fixing in the brass lever.
The lock itself has no no number so can’t date it exactly all it has is a casting pattern no in the lock body and the same number on the underside of the bolt platform. So its more than likely circa 1920-1930’s
If anyone has more info would welcome it.
