I’m posting under a different name for privacy reasons.
I would be grateful for any suggestions on what I should do next and where I stand with this.
About three weeks ago, I noticed a small water stain appearing on the living room wall downstairs. I have attached a picture taken a week ago for reference. At first it was quite faint, but over time it has gradually grown and is now spreading along the junction between the wall and the ceiling. The area feels slightly damp to the touch.
I live in a terraced house. The room above the affected area is a child’s bedroom, and the bathroom in my property is on the opposite side of the house. However, the stain is exactly on the shared wall with my neighbour’s upstairs bathroom. My neighbour had their bathtub, toilet and sink fitted about two years ago.
I initially spoke to the tenants next door, and they said there was no water pooling or visible leaks in their bathroom. Even so, they arranged for the company that installed their bathroom to come and inspect it. The tenants usually deal with most repairs themselves, as they are concerned about their rent going up, so they asked me not to contact the landlord directly. I agreed to wait for their plumber to come and inspect the issue first. When they came, they didn’t open up the wall but removed the bath panel and concluded there was no leak in their installation. They suggested instead that the issue was coming from my property.
For context, there is also staining on the wall in my neighbour’s downstairs toilet area.
Because the damp started developing mould in my property, I contacted my insurance company, and today they sent out a plumber they work with to investigate. He found that there are no water pipes running through the party wall in question, my bathroom is not located above the affected room, and my roof was completely replaced last year so there is no possibility of a roof leak either. His conclusion was that the leak is not coming from my property at all. If it were, they would have started opening up my wall today to investigate further, but he ruled that out entirely.
The problem is that the mould is getting worse, and I want to resolve this quickly. However, the neighbour’s bathroom installers have already suggested it is coming from my side, and I suspect both the tenants and the landlord next door may believe that.
My insurer’s plumber has basically said there is no plumbing on my side that could be responsible, so it must be coming from the neighbouring property, but beyond that there’s not much more they can do. I dropped a letter to their landlord today as they weren’t in at the time.
I’m now unsure how to proceed if the landlord is not willing to engage with the issue. What would be the best next step in this situation?