We share a non-public drain with a couple of neighbours. It takes the outflow from our electric sewage treatment plant, our neighbour's surface water and another house's septic tank. The drain was put in many years ago by the then landowner and flows under a farmer's field to a local river. Another neighbour has just built a new house and, in the absence of any easy alternative, now wants to tap into our shared drain for their sewage treatment plant (only). They have offered to put in a new inspection pit and say there should be no problem with capacity as the drain used to service a couple of other houses. I know that having more people sharing will help spread the cost if something goes wrong downstream but I'm concerned that disturbing the drain might itself precipitate a problem. Should I agree? Do we need to get a legal maintenance agreement or will regulations apply anyway? Is there anything else I should consider??