The stuff you've already done on your own boiler in your own home, for no financial reward can be allowable in law & by the regs if you're a 'competent person' with the required knowledge, technical expertise & experience. If told anything else it's misinformation, albeit with the best safety-conscious intentions. I am not GSR, btw.
You may well be a superb craftsman in another discipline. However, just that you're needing to ask what this leaking item is suggests you don't yet have the knowledge or experience parts of 'competent' & you could do unsafe work without realising. Bite the bullet, stay safe & get someone in.