He may have said its limescale but in reality it will be mostly debris from the heating system including metal oxides and carbonates.
You will see that the free advice given here was pointing you to a wrong diagnosis of the problem as a PCB. Whilst they can fail the cheaper options should be investigated first.
You had bad luck with your choice of engineer, I am sorry about that.
You will have noticed that most of the people on this forum suggested checking the sensors first and thats even without seeing your boiler. Most engineers are honest and conscientious and if they dont find the fault thats usually due to a lack of training/experience rather than any dishonesty.
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