Unfortunately it is poor unsuspecting customers like yourself who are pretty much left between a rock and a hard place when trying to find decent tradespersons..
There is the modern trend for websites offering "vetted tradespersons" for customers to use to get jobs done..
BUT..
what they don't tell you is 99.99% are a service that the tradesperson has to pay to join, as a vehicle for advertising to get more work..
So..
its free for the customer to search..
but the traders have to pay an annual subscription and often have to pay to get the full details of prospective jobs..
and/or pay some commission for jobs gained via the website..
This is how the "tradesperson search website" makes its money..
So there is IMHO a bit of a conflict of interest..
as they are not really independent if a particular tradesperson has a poor review..
when that very same tradesperson is paying towards their costs and profits!!!
At the end of the day, where ever possible, the good old "Word Of Mouth" recommendation is still the best option to use..
Also..
If you are a good or above average tradesperson..
you don't need to go giving some of your hard-earned money to any third party "we-check-your-trusted-traders.com.uk.scam" website,
to try and get a few extra jobs...
As word of mouth recommendations can soon fill your diary..
So those who have paid to be on these sort of websites could be new-starters....
or businesses struggling to get work for one reason or another!? :C (but that's just my hunch)
Around here.. (down in the Midlands.. {too far to be able to help you out unfortunately})..
There are a few internet social media neighbourhood sites..
things like
https://nextdoor.co.uk
and local Facebook pages..
where people living in your local area often ask for traders to do all sorts..
Plastering / Car MOT's / Garden landscape / Plumbing / Electrical / etc.. etc.. etc..
(I have had my name recommended numerous times via these types of platform...
Its free and genuinely independent..
as no one makes any money either way from recommending or not!!
Just a good neighbour trying to help others locally).
If you are not on any local to you, it may be a useful exercise to log-on and ask around locally for a good electrician.
Just to clarify.. you need someone with an "Insulation Resistance Tester"...
a "megger" is a brand name which has been generically used within the trade in a similar way that many people ask for the Sellotape..
when they actual use "clear adhesive tape".
Every competent electrician charging money for doing work.. should have access to an Insulation Resistance Tester..
it is not an unusual or specialist bit of kit.. Just part of the bog-standard tool kit of any half decent electrician.
You really do appear to be drawing the short-straw on every attempt to resolve your problem..
Sometimes a fault cannot be identified or fixed on the first visit..
BUT..
If I'd been fault investigating and not resolved a problem..
along with my invoice would be an accompanying letter/e-mail..
confirming what I have done so far..
What I have found, (proved/disproved)..
What I would recommend as the next course of action to assist resolving the problem.
An invoice with any explanation of what it refers to very poor business practice in my book!
Guinness