Thank you. The situation is not uncommon where there is only one downspout and that downspout is on one side or the other because it rarely goes straight down the boundary. There would be no point in having a downspout on each side, 100 mm apart.
If the current arrangement has been in place for more than 20 years even if the water drainage facilities are not documented, then you have acquired the right for it to remain in place under the Prescription Act because you now have an easement, the right to use the drainage.
I don’t know why they would want to simply block off your side but if they are threatening that, and it’s a real threat, you can always apply to court for an injunction to prevent them doing that. You can also ask the court to award costs against them.
It may be that your house insurance has legal expenses cover so it would be worthwhile checking to see if you are covered for neighbour disputes. You may also find that if you get a solicitors letter sent to them, it may focus their mind, that they cannot do that if the solicitor is threatening court and costs.
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