UK TT undetectable PEN fault

[Hakuhou]

What part of the electrical system does this aspect apply to?

S refers to the installation side, whereby the earth and neutral returns are “Separate” on this side.

Where do the neutral and the protective earth become separated never again to come together?

The neutral and earth separate at the intake of the installation.

Just searched youtube and found a video for you that may help with your understanding. I have not watched it through but from my quick scan through it covers one of the points you have raised:

View: https://www.youtube.com/watch?v=z7han_twcnw

This was interesting. The PEN fault at neighbor 2 meant that their neutral was returning through their earth, into the metal services, into neighbor 1’s and then into neighbor 1’s earth-neutral link.
When neighbor 1 disconnected their earth neighbor 2’s electrics stopped, with the added drama of this energizing everything connected to the earth systems of both neighbors.

To help with your consideration of this. Using your diagram in post #21.
The earth point on that 3rd electrode (closest to the installation) was moved along by some form of pixie a tad and was now at the origin of the customers installation.

I’m still thinking about what kind of earth system it would be. I think that a TT that is connected back to a TN-C-S (e.g. via some metal pipe work that’s been earthed by both systems) is an extension of that TN-C-S.

After reading all this my question becomes, what happens if the earth electrode in what is intended to be a TT system connects/is not sufficiently insulated from the CNE.
The CNE is used by other installations (e.g. other houses) as part of their TN-C-S systems.