Hayden,
Whilst it is useful if the boiler Tester has built boilers in the past, as far as I'm aware, it is not actually a requirement. He may for instance have many years experience of steaming/running/maintaining steam engines, whilst never having built one.
The competency required is that they should be able to carry out the test as laid down in the current Federation (or Association) guidelines (e.g. Green Book). There is a degree of latitude in one or two areas (testing of superheaters for instance) where an Inspector is allowed to use their own discretion – but by and large – a boiler will pass the test (or not). So the matter (to my mind) is one of being able to trust that an inspector carries out his duties "by the book" and does not allow (say) personal loyalties or friendship to influence the outcome.
Outside of the actual test, there may well be other issues arising, where it may be very helpful to get solid advice from the Tester (and most boiler testers will be able to provide this) but I'm not aware that it is mandated.
So I'm not sure how your club has an "Illegal status" if the testers carry out the test as stipulated?
I gather from another post that you are thinking of buying a 5" loco without a current certificate. The boiler is a very expensive part of the engine to have to replace, so my advice to anyone thinking of buying an engine is always to insist it has a current certificate. If the price is high, then the Seller (in my view) should have the engine examined and get a current certificate before they sell it. Otherwise, the price should reflect the lack of this important documentation.
A friend of mine (who sold live steam models for a living) told me once that (without a current boiler certificate) he simply valued the model at the cost of the castings (unless of course it was a 'display' model. Not bad advice.
Regards,
IanT