The problem is not yours exclusively; you need to phone the council and they will send their men out to diagnose and hopefully fix the problem. They will question those who use the drain which exits at your late mother's house. Someone is putting stuff down the drain which they shouldn't be. I had a similar problem, (I am an end house), and all five householders in my case, had to foot the bill equally (E1,200 divided into 5) - but our problem necessitated digging up the footpath and replacing a broken drain (our side of the main sewer). If anyone doesn't pay the contractor who was hired by the council then the Environmental health officer will prosecute. The contractor will have to get planning permission to dig up the pavement and this can take a few weeks, but they work closely with the council and it went pretty smoothly in my case. Don't spend anymore money on those private drain companies as they are very expensive - I did the same thing until I phoned the council.