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Agreed, I think my esteemed collegues had missed that you're a tenant, you shouldn't be opening up the boiler or calling a plumber, that's down to the heating engineet and landlord.
Perhaps I should have added "respectively" in my last comment. As a homeowner you shouldn't be opening the boiler casing up at all to investigate. It seem's you've received some decent advice and that the boiler appears to be viable in which case get a second or third engineer in for a second opinion.
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