I had a very similar or even probably worse situation. My neighbours downpipe was sunk into my garden leading to a soak away, again in my garden!!! Solution to get rid? Make extremely good friends of said neighbours, and come to a resultion that is ammicable to both parties involved, to the point of sharing good portions of the cost. We even we're amicable about them bricking up windows that were on our mutual boundary line looking straight into my space! So there is always hope if both parties can be realistic, adult and appriciate, what if the shoe was on the other foot! Love and peace to all..
Martyn
December 2008