The Unseen Hook includes both the first day of shooting, which was the scene in Morocco when Charles finds Sebastian, and the last day of shooting, which was the General Strike ruckus scene.
The title of this episode and of episode A Twitch Upon the Thread (1981) were taken from G.K. Chesterton's short story "The Queer Feet" in which Father Brown says "I caught him, with an unseen hook and an invisible line which is long enough to let him wander to the ends of the world, and still to bring him back with a twitch upon the thread".
The "general strike" in this episode is the strike of 1926. It lasted nine days, May 4-12, and was an attempt to force the British government to prevent pay cuts and worsening conditions for British coal miners.