Well, in my opinion in specific a manufacturers engineer (was he?) should at least follow the written instructions of his own employer.
On the other side I do see the BG-type of service as well as the WB-quick service as a total legitimate way of doing it as they do provide a guarantee with it.
But your neighbour going about servicing boilers should not try to accommodate their way of working as he does not provides the customer with a guarantee on the boiler function. In this case there should be a complete service the only thing a customer can expect.
Not to forget BG as WB sit in the board making the rules for us. If anything would come across they can legalize it to an extend. Does Joe Plumber have the same possibility? Not exactly.