Just thought I'd let you know what happened on this. I had a plumber and a builder out both of whom confirmed that next door is on a different spur/pipe to our rental house and that there was no way the leak could be coming from us. They didn't give me a written report though. Just confirmed on the phone/by text message.
The lady next door had a scribbled invoice saying "that'll be £x. It's coming from next door" which apparently I had to take as gospel.
When I suggested we go halves on a formal report she rejected this.
She demanded a formal investigation from me, but apparently no need for her to provide any evidence beyond a scribbled note that it was coming from my side.
She then went beyond repeatedly calling and hassling the letting agent and my lovely tenant (who was amazing throughout and I've sent a present to for all their help).
She called the police on me - great.
She emailed her local MP about how evil the evil woman who owns the house next door is.
I get it. I must be evil. All I could think was that she must really believe what she's saying. And I felt for her a bit. She lives in a terrace in Accrington. She won't be really well off. I thought she must believe it.
So anyway at my wits' end I paid nearly £600 for a formal report. My lovely tenant took a half day holiday to let them in. The date was confirmed with the lady next door before the booking was confirmed. Payment up front etc etc.
The morning of the appointment I got a call from the plumber. They were in next door (kids at the window) but refusing to answer. The tenant knocked, the agent called. They kept on trying. Nothing
So the upshot was she can call the police on me and tell her MP that I am a piece of work (the MP's assistant responded saying that she should call United utilities to do the works and then bill me for it. Helpful).
She made me spend hundreds on a report. And then she refused to open the front door.
What can you say?! Got to chalk it up to experience and hope we don't hear from her again!