Thanks Woody. It was an idle thought really, on the back of thinking, "PP was for three, but the third was never started so will have lapsed." Then I considered that it was a single pp for 3 dwellings, and seeing that 2 had been started (and finished). Hence the question, "How long can you string out a construction... can you be forced to finish or halt?"
To respond to your last answer, it seems to me that the pp WAS enacted; unless a planner wants to argue, "Ah, but you had to start all three!" Which is unlikely I'd have thought.
I can't afford the one whose garden the third plot would be in, so it's just a thought experiment.