Your best bet is to buy a freezing kit, put a stop tap in a suitable place for future maintenance whilst it's frozen.
Boiler parts are priced in the same fashion as car parts, ie; expensive.
I would need to be in front of your boiler to diagnose the exact fault....
These are great boilers, they must not be used with the cover off, please get someone in who knows what they're doing to sort it out for you.
Sounds to me like it's just pulling on the trap.
AAV needing fitting on trap maybe?
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