Be very careful then. We've been in a similar situation, and our solicitor advised us that firstly we shouldn't allow anything to be done without written permission, and secondly not to give such permission!
I'd be having a wee look over the weekend at where that pipe goes to, and what's coming out of it. It sounds as though it may be the overflow from an existing septic tank. If they want to take the overflow from a new tank across the farmer's field and into the burn, that's one thing. Having a soakaway on your land or next to it is not the same thing, and is something I'd try to avoid if at all possible!