I'll try but it's a trade on its own personally speaking. It's an expensive trade if you get it wrong. Cheers for your advise
As last Plumber intimates, in the old days, we learned the full range of activities. I started with the old Gas Board in'76. I did electronics (PCB's were starting to be introduced to gas appliances, pipework, cookers, fault finding on CH and boilers, wall heaters. We were trained on everything gas, including small commercial package burners.
Even BG now do not train their guys in anything except boiler fitting and fault finding, and even then i understand that they are one or the other.
What is your company's core activity? I am guessing it is not plumbing or gas fitting? Your boss is a dick if he expects a new guy to understand everything. He is clearly lacking in understanding himself.
Maybe you should look for another firm? You are getting paid peanuts, so you may find someone local to take you on at similar rates, and allow you to develop.
Do some reading, and get old boilers and parts and strip them to understand what is going on. Look for another site (google my username) which has a massive wealth of information.
Meanwhile:
Boiler Servicing Procedure for Heating Engineers and Plumbers
Central Heating Fault Finding and Fault Repair for DIY Enthusiasts | DIY Doctor (scroll down a bit)
Try and get your boss to stump up £200 for the Baxi course. If he is reluctant to appreciate its value, tell him to google,and he will find a LOT of guys recommending it, and maybe he could talk to the Baxi trainers.
TBH, if he will not allow you to develop, and you do not move, you will end up leaving the industry, which will be a shame