True, We can not see what has been done some time ago, but if Joe Bloggs fitted it a year ago, I would not be faced with something I already knew or suspected to be an installation by an unqualified person.
Every job carries the risk that there is an unseen bodge.
A recent one was a Brittony water heater in a kitchen cupboard. Clearances to the sides, top and bottom were insufficient, but the clincher was the neat little holes someone had cut in the room sealed casing to make room for the cupboard door hinges!
The house had recently been sold, so the buyer had no knowledge of the person responsible.
Another was a combi that vented into a carport. Not a good idea, but then the householder decided to put windows and walls in and turn the carport into a sunroom. They called me in because the boiler fan was noisy outside. They didn't think it was any of my business that their flue had been rendered unsafe and illegal by their building work, and said that as it had been done a year ago, it must be alright. Like many other of us, I must have missed dangerous bodges...... Like the British Gas blokes who fitted a water heater, for an elderly lady in almshouses, which I was called to for a smell of escaping gas. BG had failed to solder two 22mm copper elbows in a meter box. they had fluxed them, but forgot to apply heat. And failed to find the leak when called back.
We can not guarantee to always get it right, or find everything, but we can refuse to condone illegal work. So, yes, I see your point, I might service a boiler installed by Joe Bloggs, and indeed i might find it excellently installed, unlike some "professionally" installed examples I come across. And I would never know.
So, I might do it unknowingly, but I stick by my earlier statements.
And, ref Oilman, you are so right... Corgi registration is not very well understood, It costs us an arm and leg, we have to redo all our exams at great cost every five years, and can't fix grannys boiler at the weekend if we're registered as employees of a firm, unless it's a company job, or we risk five years in jail for doing what we are doing every other day of the week.
In what other skilled trade or profession are you deemed to become incompetent at the stroke of midnight five years after qualifying?