I had a similar situation. The toilets in the house, two doors down from me kept over flowing. When they consulted a plumber, it was deemed that the cause of the problem was the tree in the front garden of the property I am renting. So after notifying the rental agency (Foxton's), the neighbor went ahead and had work done to remove the tree roots that were causing the problem. All four house that were connected which shared the drain, ended up paying 40 pounds each. It seemed like the reasonable thing to do. And thankfully, this neighborhood did check before going ahead with the work.