Ferroli boiler - Flue seal leaking - I think

Our boiler was fitted in 2007, it has been dripping from the top, coming down the flue through the ceiling, causing some plaster to come down. Ferroli won't accept any responsibility as they say it's installer fault. Installer says it's wear and tear (first contacted 2 and half years after installation).
Ferroli engineer says the job needs to be done by general maintenance men, but they say coz it's attached to the boiler they won't do it.
The outside has been resealed by my father in law in case it was coming in when raining, but has made no difference. The flue runs across the whole of the flat to get out so means the ceiling will need to come down.
No one seems to be able to help us with this, it's making our flat smell damp now and I'm worried it's going to ruin the boiler.

Can anyone help? I just want it fixed so I can decorate and stop stressing about it.

If nothing else, you need to find a solution to have access to the flue over it's entire length. Other than that, it is hard to comment without seeing it.

so what you are saying is you have a flue concealed in your ceiling void and its leaking.
STOP USING YOUR BOILER.

you need a different engineer to look at this immediately. if your flue cannot be seen along its entierety and is though to be leaking, you or others may be at serious risk of carbon monoxide poisoning.

turn it off and call a reputable engineer out.