I would suggest you should purchase the clamp meter, it is invaluable in the situation you describe, it will give you an instant guide to the problem, just put it around L+N (not around the cpc as this will not give you all the leakage).
I had one similar a couple of weeks ago, customer phoned WPD and said the was access to live parts as the wooden shourds were missing. They were there with 2 hours, new cutout and MET fitted FOC
You are correct, the cable isn’t big enough for in insulation.
As this is notifiable work I presume that the contractor is registered so you can complain to the body ie. niceic/napit and they will do nothing.
good luck.
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