I loved my loop impedance meter, however they are not all the same, with the 13 amp plug leads one meter would measure line - earth for ELI and also line - earth for prospective short circuit current, but the other one switched to line - neutral for PSCC. But one got a feeling for what results to expect, not a case of if not 1.38 Ω or less there must be a problem, but this socket is only 3 meters from that one, why is the result so different.
I well remember testing a board in the C&G 2391 exam, the CU was showing 1.10 Ω and socket less than a foot away 1.20 Ω so I investigated further and found in the cable a 0.1 Ω resistor, clearly a C2, maybe a C1 as if the socket was used that resistor would likely go on fire.
It had been put there not as a fault, but so the board emulated a large premises, however my finding the resistor presented a problem for the lecturer, should he fail those who did not find it? Don't know answer, however my point is when using test equipment day in day out you get to know what to expect, and so readings within spec can still ring alarm bells.
I know a house the size of mine, I can expect to read 0.024 amp leakage total, but with a caravan looking at 0.002 amp, but in both cases the insulation resistance over 1 MΩ.
Get a new instrument and it takes time to work out what one should expect, I have a non contact volt option on new meter, I know from using it that it will not show telephone cables as live, but until used, was not sure if there would be enough of an AC component from the dial tone to show on meter or not. But it shows my laptop as having voltage, and that also has a DC supply. Only by using it does one work out what to expect.
I hope we do get a report on what was found. Faults can go for decades without being found, this house had a borrowed neutral at the hall light switch, could have been like that from when house was built, only found when all RCBO consumer unit fitted. Should be found with an EICR, I would test now, but not sure if it was tested all those years ago when I did my C&G 2391, it was 20 years ago. But I remember trying to find one in a Bank, and in the days before we walked around with clamp on ammeters it was not easy.