If the plant belongs to you and you have failed to maintain it and as a result of that failure to maintain, it has grown to such an extent that it causes damage, then you are liable in common law negligence.
The fact that there may have been no incidents before or any complaints before is not relevant.
With regard to the damage, you would be liable for that damage although you may only be partly liable if the owner of the property is simply stood by and let this grow, watching it, until it caused this damage. Whether that would be the case would depend on whether he could reasonably have inspected the extent of the growth.
I am not a boiler expert so I don’t know whether this plant blocking the flue would cause the kind of damage he describes or whether that is the kind of cost that would be necessary to repair it.
It may be that the plant blocks the flow either in whole or in part but it may be that the boiler has broken regardless and he’s trying to blame it on you. In my experience of modern gas boilers, if the flue gets blocked, the boiler simply goes out to make sure that people do not die of carbon monoxide poisoning.
If the neighbour wants to claim the cost of the repair from you is going to have to prove to the satisfaction of the court if everyone gets to court, that the damage to the boiler was caused by the plant. Just because he comes up with a letter from a friendly plumber which says that would not necessarily convince the court that is the case particularly if you came up with something from an independent expert which said that under no circumstances would this kind of blockage have caused this problem.
Whether his claim fails or succeeds would depend on the evidence he obtains, the evidence you obtain, and the view that a judge in court would take. You might want to pass it on to your insurer although for the sake of £400, you might not want to risk losing any no claim bonus on your policy or the premiums increasing next year. You do not have to pass it on to your insurer. If you steadfastly refuse to pay, he will have no option but to take you to court and you will have no option but to defend those proceedings.
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