Obvious use of stunt double for Steed during fight at the stables.
When Mrs. Peel is exploring the dark stable, she lights a kerosene lamp. At one point, she is holding the lamp in front of her, but her shadow is still on the wall in front of her. Obviously, the scene is being lit from behind her.
Early in the episode, they show flocking birds, and then individual birds dropping to the floor dead. One of the 'flocking birds' shots is actually film of fruit bats, native to the tropics, not the UK.