Not only is this episode very similar to the hour-long Fish on the Hook (1964), but one off this episode's actors, Zena Marshall, co-starred in that episode as well.
When Nikolides takes Drake to check out Ranfi's villa (at about the 7 min 45 second mark), the villa is, in fact, the central piazza and Bristol Colonnade at Portmeirion. The is the second time in the series that Portmeirion - famous as the location for The Village in The Prisoner - makes a guest appearance.