I explained to the customer perfectly well, prior to the upgrade and again in detail on my return visit. I also did not charge for the call out and the 2 replacement lights switches and wall light I did as a good will gesture during the initial job of swapping out the CU.
He was clearly not buying what I was explaining to him - that the fault was most likely already present and that the new RCD had picked this up. If he had been polite to me rather than ranting, and accepted my explanation regarding the oven fault then I would have been happy to replace the faulty element (probably at no cost) since it is was a straight forward job.
So, sorry no I don't agree with you and take offense on your comment about "poor customer service". I have been in the trade for over 30 years and the only genuine complaints I have received in all that time have been for poor or faulty equipment failures, which I have either replaced, if I had supplied them, or been happy on most occasions to replace, at no cost, for decent customers who have accepted that the accessory/equipment they have supplied is faulty. I have and still do, go over and above for good customers and as a result have a very long cliental list of repeat customers and recommendations.
All the other complaints (like the 2 examples in my post) were people who either were not willing to understand/accept the cause/situation or were chancing a compensation claim of some sorts. we have all been victim of this I'm sure.