Oh that is disappointing. It must be incredibly frustrating, Chris I was hoping that you would have Anna back to her old self by now and could enlighten us as to the cause of the problems. It seems that you are no further forward and are still in a position that you either have to start replacing parts to see what fixes it or to find an engineer who can identify the problem.
I assume that by boiler inspection he was suggesting a boiler safety inspection rather than checking if a descale is necessary or a visual inspection for leaks. From a safety point of view the most important item is the safety relief valve. With a copper boiler corrosion should not be an issue only serious mechanical damage would compromise the safety of the boiler. In a home situation you would know if you had subjected the boiler to such damage. A simple visual inspection may be useful to find leaking fittings but these would not compromise the safety of the boiler. The safety relief valve can be removed, cleaned and inspected or simply replaced if you have any concern that it could fail to open.