This car has been puzzled before but without a lot of the background information, which you have now provided so it's a point for you!
Actually what I read was that Charles Ward wanted all the trappings of the Bentley that he couldn't afford so did everthing on the base of this MG, but whether that was so at the time or whether the Bentley bit has been added to the folklore later I don't know, hence your info (especially the "Symphony" name) is enough to solve the puzzle.
Maybe that extra name could be added to the puzzle title?