Pity the BSA Twin was not in the main line up, it's there in the background. Assume it's a Super Flash. Maybe too heavy for the gal on the Bantam. Matchless is an early swing arm version, so pic from around '55? Earlier models were rigid frame, tele forks.
Bike parked behind the Flash, half in view, could almost pass as a modern rice burner factory custom.
At least one imported motorcycle has marked its territory, on the paving in front of the line up. The strapline on the Valvolene ad could be the answer to wet sumping, but as always, you can't believe that claim.
Swarfy.