Thank you both for your feedback. I will get on the case urgently (by
removing the fuses, I meant that I have cut off both lighting circuits
until I can resolve the issue - I am however still running power
through one rubber insulated circuit - see below).

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

Thanks again for the advice
Harry