Back again with some answers.
Fortunately, granddaughter also comes with son able to assist in this project, which made matters much easier. None of you gave me the obvious advice that any faults would be found in the places most difficult to access! But so it transpired.
The first post from EFLImpudence gave the correct diagnosis; namely that there were two faults: a break in the earth wire and a N-E Fault, (in different places on the system). However, finding them both proved far from easy.
The broken earth was in a twin and earth cable that has bee untouched for many years. Probably this fault would have remained undetected if I had not installed the RCBOs and then encountered the strange symptoms I described originally. I suspect that the cable could have been damaged by a spike from a carpet gripper, which may even have been the source of the high resistance to earth reported earlier.
This cable went through a wall to a JB which was located behind a glass fronted display cabinet containing all of the wife’s valuables and carefully secured to the wall. Moving the cable off the carpet spike restored the earth continuity, but I replaced that length of cable. This solved problem one, but meant that the RCBO would trip on both halves of the ring, so I simply had to find the N-E fault.
Despite suggestions to the contrary on this forum, I could think of no way of finding it other than breaking the ring in various places and using a long wire on the multi-meter back to the CU. The final point that needed to be isolated was a JB with two spurs at the back of a low attic. Fortunately, son, who is into caving, managed that after we had removed a few hundred books.
The culprit turned out to be a relatively recently replaced twin socket, the fault disappeared when the fixing screws were loosened. On inspection, it seems that the earth wire, (multi-strand, as used in the original house wiring), had bent in such a way that the sleeving slid back and permitted the earth conductor to touch the neutral. You could not do that if you tried!
We found no incorrect wiring. After another couple of hours reassembling the ring, reinstating cabinets, books etc. the system was retested for insulation and put back into service, hopefully safer than before.
Many thanks to all contributors who helped solve this odd problem.
This has shown me the value of RCBOs over RCDs and MCBs, but it has also shown me that this forum offers a lot of expertise to the DIYer, even if he is a Chartered Electrical Engineer who thought he understood domestic wiring!
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Incidentally, when I said 100 ohms; that was meant to imply approximately. If I had meant anything else I would have put 100.00 ohms, as per my NAMAS training! As it was measured on several ranges of the ohm-meter, the digital restrictions did not apply.
I apologise fro my casual use of the term Megger. It should have read 1000v 2000MΩ YEF YF509 digital insulation tester. I think these are now banned, because you could turn them into TAZERs – such is life.