In the excerpt from "The Dark Game: True Spy Stories from Invisible Ink to CIA Moles", by Paul B. Janeczko, the sentence that best states the controlling idea of the passage is Caution and precision enabled Wyke's cable-tapping success.
The narrator describes all the troubles that Wyke faced to reach the cables. Then he states that "...the work called for a deft hand...", and that Wyke was "... the "SIS's phone tapping agent." There is a description o how careful was Wyke with his work and how skillful he was.