Hi Westard. I've been back to this installation again.
Re "Testing an existing circuit you should have your neutral conductors disconnected and linked to the line conductors, keep the cpcs connected" - do you mean that at the board I leave the CPC connected and remove the line and neutral conductors for the relevant circuit and connect them with a jump lead? So as per the attached illustration?
Once I'm clear on that and I'm back at the installation I'll perform the test, but in the meantime I corrected a couple of mistakes I realised I had made with the wiring and retested as per post 14 and have different (better?) results:
Neutral to earth: >999
Line to earth: 0.31
Line to neutral: >999
As I understand it my Line to Earth is my issue as less than 2 mega ohms is a fail.
I've been around the ring circuit unplugging everything and identified a few accessories that I hadn't previously been aware of: a spurred electric radiator, its switch (which has a couple of neons), a socket with a USB port. I disconnected all these and - to maintain the ring - joined the relevant conductors (earth to earth, line to line, neutral to neutral) together with jump leads / wagos (which I presume was the correct thing to do???).
This made no difference however, the reading remained at 0.31.
One theory: 4 of the double sockets have tiny neon on/off lights on the switches - could that be responsible for the reading? Or am I simply overlooking something that's still in the circuit?