I looked at his business card - he is also a member of IDHE (now called
IDHEE) - "The IDHEE is still the pre-eminent professional body for the
domestic heating engineer. The Institute aims to promote energy
efficient domestic central heating components and the installation of
safe and efficient systems. "

and also a member of IPHE - Institute Plumbing & Heating Engineers .

So I would say that from a professional Standing point of view he fits
what most people would expect as 'meeting the standard' but as I
mentioned in previous post, I did not follow his advice and used
Inhibitor.
I had installed the system myself, and he was commissioning the system
and issuing the gas safety certificate.

However somebody who is not well versed with heating systems, may well
follow the advice of "the Professional" and not use the inhibitor.

Rick