Rick wrote:
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.

He may be competent about every other aspect of heating systems but I
strongly disagree with his policy of not adding inhibitors; it must
cause considerable long-term damage to his installations, but the
warranty would be long expired before the damage was noticed. A
ha'porth of tar. I wonder what the IoP (now IoP&HE) Guide says; I may
have to look.

told me he NEVER adds inhibitor, it is
not needed and just a marketing ploy.

There is ceratinly an element of snake-oil marketing involved. No water
treatment company will volunteer to tell you the ingredients of their
solutions. I think you could make an effective home-brew inhibitor for
less than £1 a go, IF you had access to the chemicals.
They charge an exorbitant amount for their test kits, so most people
cannot just top-up as required, but tip in a whole bottle at £15 a go,
to be sure.

Excessive pH (9) can damage copper pipes I believe, but I've never
seen it happen.

You can do an interesting experiment about galvanic corrosion by
connecting a multi-meter between a piece of copper tube and steel pipe,
with their ends in a bucket of water. Plain mains water will cause a
very small PD between the two pipes. Adding a few drops of anything
acidic (lemon juice, vinegar) can increase the PD by a factor of 10.
Adding anything alkaline (caustic soda, oven cleaner) will reduce the
PD to 0.