Why is it so hard to find a reliable Plumber?

We bought a house as an investment rental property in January 2016. A new boiler had been fitted in December 2015 so we were fairly confident that we would not be incurring expense on the boiler for a few years. We had used an agent for various activities on the marketing and letting process and their service had been good so we decided to use them for the gas safety check and boiler service which they carried out in Dec. 2016 and Dec.2017.

On Saturday the boiler failed completely the main heat exchanger badly corroded together with the secondary heat exchanger . The plumber confirmed that the filling loop had been installed incorrectly so mains water had been entering the whole system for 2 years . Other installation errors were also apparent albeit they would not have caused the boiler to fail.

He removed the magnetic plug and said it was full of debris from corrosion and probably because the system had probably not been power flushed on installation. It was the worst he had seen in 21 years .We now face a bill of £3451 to replace the boiler and rectify the faults. The boiler manufacturer’s engineer came out a year ago to change a faulty gas valve. So 2 services and a manufacturers engineer attend to the boiler and none of them check the magnetic plug or spot that the filling loop is incorrectly installed . Am I missing something here ?

The agent’s response is that it assigned 2 different engineers for the services ( which in my opinion makes their standard of service even worse ) and that it would be useful to have more clarity around boiler services .

Can Property 118 not develop a quality service and resource it to stop this sort of thing happening ?
Which conducted a survey recently and confirmed 40% of boiler services are inadequate and miss basic but in some cases critical items.

Rant over but any suggestions would be gratefully appreciated.
And yes I am writing to the boiler manufacturer next.

Christopher