On Thu, 12 May 2016 08:26:22 +0000 (UTC), Lobster
wrote:

Recently had to have my dead boiler repaired. and was set to have the main
PCB replaced with a (self-sourced) reconditioned one rather than a brand
new one at an eye-watering price.

In the end the problem turned out to be something else; however my point in
posting is that the engineer tried to warn me off using the reconditioned
part because he says that he's heard of somebody having had a house fire
caused by one,

Probably the safest place to start an unintended fire in a house would
inside a boiler. Other than in the combustion chamber they have very
little which will burn and a steel case to contain anything which
could. A PCB failure, no matter how catastrophic, would not cause
anything more than smoke and sparks.

followed by the resulting insurance claim being turned down
due to the boiler not having been properly repaired by a standard
manufacturer-supplied part.

Standard scaremongering by boiler fitters (they are not engineers) .
From a purely practical point of view there is very little visible
difference between a "new" and refurbished part and certainly none
after either had been in a fire. It is highly improbable that the
insurer would know the part was refurbished or would care. Fires are
not inspected that closely unless there is obvious evidence of arson.

A fire is plausible I suppose, but as to the last bit I smell bull****.
Could it be any way legit for an insurer to reject a claim on the above
basis (unless presumably they had highly specific and restrictive small
print in their policy wording?)

No, insurance contracts used to be "contracts of utmost good faith",
the purchaser had to tell the insurance company anything the insurer
might consider to be relevant (but the insurer didn't have to say what
they might consider to be relevant). This created significant
problems as insurers would try to use non disclosure of unrelated
factors to cancel policies after claims were made.

This changed in the Consumer Insurance (Disclosure and
Representations) Act 2012, which came into force in April 2013.

http://www.legislation.gov.uk/ukpga/2012/6/enacted

This replaced the traditional principle of utmost good faith in
consumer insurance contracts with a duty by the consumer to take
reasonable care not to make a misrepresentation to the insurer.
Basically the insurer now has to ask specific questions and the
consumer has to answer them honesty but they no longer have a general
duty to try to guess what the insurer wanted. An insurer cannot void
the insurance contract because they were not told something they
didn't ask about.