The situation in this thread is becoming clouded by supposition.
Under EEC rules all commercially produced pressure vessels must carry a test cert and a CE mark at point of first sale.
Any pressure vessel of less than 2 bar/ltrs does not require any further testing, even if sold secondhand/used. (that pretty well covers everything we use in this gauge)
Over-testing can actually damage a perfectly good boiler, which is why copper boilers on models (over 2 bar/ltrs) are now only hydraulically tested every 4 years, to 1.5 times their working pressure, though they do still have a steam test every 12-14 months.
Testers do not require any formal qualifications but Model Societies have a duty to appoint someone who is competent and with some relevent experience.
These regulations where last revised in 2008, in consultation with Midlands Fed of M.E.s, the Northeren Assoc of M.E.s, 7 1/4" Ggauge Society, and the Southern Fed of M.E.s, Ground Level 5" G Mainline Assoc, H.S.E. Model Steam Road Vehicles Soc, representatives of the trade a model maunfacturers, and the Royal, Sun Alliance Engineering representatives (which pretty much covers anyone with an interest). Further consultation is continuous, with regular meetings.