Sorry I'm no help here David. I'm just interested.
You might know that, in PME systems ......... all that comes into the property is a phase live and a phase neutral. They strap an earth to the neutral to give you an earth. N and E are entirely separate around the installation,
With the N and E effectively connected in parallel around the house, it follows that each can mimic each other. I'm not explaining this very well. I'll leave it to BrightSpark.
Apart from all the "funny" things that can go on with borrowed Neutrals etc, it has been known for neighbouring faults and "idiosynchrasies" from next door to show up with PME ......... in extreme cases........... because of the shared N/E
I'm out of my depth with 3-phase, but I'll bet it has something to do with it, as BSpark says.
In a street supplied by PME, any of the houses could be at different phases, so if a connection, or an N is "borrowed" from next door............. all kinds of silly things can happen. I just remember my tutor telling me all this .......... we left it at that though lol