I cannot imagine that someone can use your land for water waste disposal.
It amazes me at what people are capable of doing....to their own neighbors, no less.
I am not sure what a Party Wall is, but I doubt a permit was granted for such work. Yes, report it.
Listen to what happened to me. On one cold winter evening, we noticed something very strange in our yard. There was this odd and eerie mist lingering in one section of our side yard. We walked out there and I began to smell something familiar as we approached the area which had a bubble like substance on the ground below the mist. That's it--Tide Laundry detergent, that's what I smelled!
Upon investigation, I was shocked beyond belief to find that my neighbor had buried, under leaves, a 4" round PVC pipe with holes drilled into it, leading to the bushes on our property line. Attached to that PVC pipe was a section of flex pipe that lead to a hole in the neighbor's house. They had illegally hooked up a washing machine and "plumbed" the water to be disposed in OUR yard!! The NERVE!!
I figured it is not even worth discussing this issue with the neighbor because anyone so ignorant as to do such a thing is going to be impossible to reason with. So, I decided to use actions, not words. I disconnected the PVC pipe and heaved it over the bushes into their yard. Then, I went over to their property and moved the open end of the flex pipe to their porch and pointed it at their front door, held down by a rock.
The issue was never even mentioned or discussed ever again!! LOL