Thank you Jim for your helpful and constructive response.

Your building trade analogy interests me. The last but three electricians I had in left me with a non-earthed installation, a plug socket that fell off the wall more or less the first time used, safety signs saying 'do not remove' littering the floor and other faults. One of their men spent a lot of time on the phone discussing the 'trip' he had been on the night before, which no doubt accounted for the hallucinations he was describing as well as his very red eyeballs. The next to the last electrician I had in did a proper inspection of the installation and compiled a list of problems to be put right by the last lot I had in (the last but one was going on holiday). So when I think of the building trade, I think about issues of reliability and confirmation and evidence.

On the positive side the three sets of wire cutters left around by the last lot but three are useful for trimming strings when re-stringing musical instruments.