Take your head out of the clouds, this would have been picked up at at service.
An combustion check alone would have picked it up.
There's an old corgi tech bulletin about the insulation on these boilers must be checked,
Strange how this gas eng knows to check it?!
Well done on the diy gas repairs.
Im surprised you didnt find this fault or even better prevent this fault form happening when you had the boiler serviced correctly?
Another example of why people should have boiler serviced correctly once a year and leave gas work to people that know what...
To be honest best thing you can do is get a full time plumbing apprenticeship.
This should take 4 years full time Low paid employment.
Then prob bout 5-10 years experience on the job learning fron your mistakes.
Then go for it!