Watching this just now, I'm amazed that no one else has noticed the strong similarity to the feel of the Patrick McGoohan series, Secret Agent (AKA Danger Man). From the very start, I kept telling my wife that this episode was making me miss Secret Agent. As the show went on, I kept thinking, this really is something you could drop John Drake into easily.
Looking at Brian Clemens writing credits, he was involved with ten episodes of Secret agent, five of them as a writer. The similarity can't be coincidental. I'd have to guess that this was a Secret Agent plot that was either un-produced or perhaps rejected for some unknown reason. Edd Byrnes also has some similar facial features and expressions to Patrick McGoohan, making it all that much easier to imagine this is an episode of Secret Agent from the 10th series had the show run through 1975.
So if you're a Secret Agent fan and you haven't watched Thriller, you really should watch this episode. Amazing.