Back in the days when BOC Entonox demand valves were ubiquitous, there was a quarterly minor service routine, followed by a major service once a year.
The major service (and, indeed, the one taught at Falfield) included stripping the unit and replacing certain parts (although the pipeline version was simpler - no pressure reducing valve). The famous A-frame Test Bellows was required
Who knows what is "permitted" these days! I guess you could check the plastic casing for cracks and (or) deterioration ... and also check the Bodok seal and contents gauge if you come across any connected to a cylinder.
It sounds like the plan you mention, plus a quick functional test, should suffice. Have you ever had one fail?
