Unfortunately CORGI or whatever registration is almost completely unrelated to any ability to diagnose problems such as you describe. There's a little bit in the ACS assessement on gas controls (they've dropped gas relays, which probably haven't been used since they invented electrickery, but were quite fun, and have introduced modulating gas valves), and one's supposed to be able to test that safety controls such as APSes work, but it's basically about ensuring that gas and POCs stay in their assigned places rather than fixing stuff that's broke but not dangerous.
Even so one would hope that one out of 3 RIs would spot an APS problem.
Heh. Had a nice one this evening[1]. CH system blowing fuse in the FCU when switched on, and tripping MCB on the CU when customer replaced the 3A with a 13A in the FCU . Apparently he'd had a plumber out who pointed out the water dripping out of the switchmaster 3-port valve and suggested that was blowing the fuses and that he needed an electrician. He got my name from a neighbour I'd changed a Suprima PCB for so I don't know why neigbour thought I was a sparks but wtf. Customer was running the DHW off the immersion (because he thought it was better, hmmm) so I thought I'd just bypass the valve to get the boiler to fire up for CH, but of course a switchmaster's a different wiring from a Honeywell etc and I got a little confused. Customer helpfully offers that he's found something about the Switchmaster on th'interwub - guess what? The article from the wiki, wot I rote :-). Which helpfully had the wiring colurs for the valve so I was able to rig it to work the system. Good old me, eh?!
[1] yes, Saturday, and I only charged 2 figures, more fool me