Posted: 20/01/2017 at 21:21
I'm with Nut on this. A couple of corner bends from the plumbing store, and divert it back to his side. Glue them in well.
I think that legally, it is classed as a nuisance.
Nuisance in English law is an area of tort law broadly divided into two torts; private nuisance, where the actions of the defendant are "causing a substantial and unreasonable interference with a [claimant]'s land or his use or enjoyment of that land",
Maybe a letter from a solicitor if you don't want to take direct action.