I've installed loads of drains gradient, invert level & chain. Also fitted out commercial boilerhouses 150/100mm welded, Skools, Hospitals etc. Fitted all types of boilers, all sizes & fuels.
All types of Domestic & Commercial Plumbing/Heating work. Roof Leadwork etc too.
Covering most types of work my trade requires.
So you are the right guy who could in fact help OP, install a couple of Co detectors in the duct void, since you appear to have a vast range of experience in many other fields, using two Co detectors, to make it double safe, in case one detector fails to respond, and wire it to a small control panel that disrupts power to the boiler, what is so big an issue with this, you are not actually drilling or cutting any flue or touching the boiler, you are just adding a co detection system. It has nothing absolute to do with gas safe rules, and as for inspection, from the flue terminal, where his joint is only 300mm from the outer wall easily seen for inspection, so where does an issue arrise from since you are not physically doing anything to the flue, you are just enc;losing it in a duct and making provision for inspection as well as having co alarm fitted within it.
I did this to my rented property where my warm air unit was I was told more vulnerable to Co leaks and with lethal consequences as warm air could reach every room in a house within seconds, so instead of having a Co detector just sitting near the WAU cupboard, I took its output and wired up to a relay that tripped power to the WAU, how complex do you think it can get, and it had absolutely **** all do with gas safe regulations. ( with due respect to Gas Safe Regulations)
BTW, my tenants were absolutely ****tards as they kept taking the batteries out for their children's toys, thickos, so I had to even make a small mains powered DC supply for it which also ran the latched relay. It never required batteries, and in case of power fail, you did not need battery back up as all else would stop electrically. In practice the Co detector never ever detected any leaks, so it never tripped power to the WAU, and sadly most gas safe engineers were not too keen in continuing to service it due to its age nearly 30 years in service, so I took an opportunity when those irresponsible tenants from hell were evicted and I replaced it with wet combi boiler and radiators, that WAU is still left intact ready to fire up if need be, as a back up heating.