To be fair to them, Conn, they may well be perfectly genuine. So much with fixing faults on modern boilers is a matter of trial and error. Whereas with a car fault, it's usually localised, presenting an identifiable problem, so many boiler parts are interdependent. One cheap little thermistor could give a fault that appears to be with a major (expensive) component.
Not a perfect state of affairs at all. The only answer is to find someone with experience of this particular unit. Someone who can say "oh, I've seen this problem before. It's usually the .... etc )
Strange and "irrational" faults are often caused by a problem with the circuit board for instance. The trouble is, once a part has been substituted, it becomes second-hand.
Keep plugging away at trying to find an experienced engineer. I would contact the manufacturers for names of qualified installers/repairers in your area.