Harry wrote:

The one ring main circuit which has rubber insulated wiring passed the
insulation resistance checks easily (the electrician said three times
the suggested minimum), and has caused no problems.

I would not take too much comfort from that. The minimum is 250K ohms -
with decent insulation you would expect a reading of at least tens of
meg ohms with modern PVC insulated wire. Rubber when new would have
probably been higher still.

In spite of this,
should I still be seeking to replace it, and how urgently?

Yes, within the month.

--
Cheers,

John.

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