So let me see if I understand you right:

1. You have a problem with your boiler which turns out to be a fault which
the manufacturers have a free fix for, which engineers who specialise in
your make of boiler should know about.

Correct. You've got that bit right.

2. You engage an engineer who isn't a specialist in your make of boiler and
who doesn't know about the manufacturer's free fix, who proposes to charge
you for doing a repair on the boiler as he would if it were any other
boiler repair job (including asking you to pay for him getting and fitting
model-specific parts for the appliance which, not being a specialist in
this make, he couldn't reasonably be expected to keep as van stock).

Not correct. I did not engage an engineer who isn't a specialist. I
contacted Vaillant and booked a visit with them for one of their own
engineers. Apparantly, because they were busy, they delegated it to
one of their sub-contractors who then engaged a sub-contractor of
their own - and he was the cowboy. Got it?