The x ray of the Thingumajig is actually the x ray of the case in "Take Me To Your Leader".
When the fisherman first approaches the water, just before he discovers the dead fish, a large brick bridge can be seen in the background. This is the same bridge that Tara runs across in the title sequence (Tykes Water Bridge, Aldenham Park, Elstree).
Linda Thorson later confessed that she was sick during the shooting of the episode, and she had to remain in her flat. So the producers replaced her with another actress to stay with Steed.
The 'generator' in the tunnels is an engine driven welding machine.
One of the rare episodes where John Steed character doesn't win against a villain in a fist fight.