Hello I am a solicitor with over15 years experience. I will try to help you with this.
It is totally wrong and the landlord has the responsibility to fix this.
His repairing duty is in s11(1)(a)landlord and tenant act 1985.
Yes you could do that. What sort of tenancy do you have?
Sorry I thought you were the tenant. So you are the owner and the neighbour's house is tenanted and causing the problem in the way you describe.
The answer I have given was wrongly premised. S11 LTA does not apply. The situation is not straightforward. The use of his land is causing you problems which if they are foreseeable will be his responsibility in negligence. However, if the properties were as they are now when you moved in then it could be argued that caveat emptor applies.
You should get a quote and if it is not too much then just fit the membrane and avoid litigation entirely. I am sorry I misunderstood your question.
have you asked the same question twice but with figures in the other question. The short answer is that you mayhave the legal rights on your side but at this level ofcost it is not worth going to court over it.