If you have a drain that is shared with your neighbour, the water authority will be responsible for the shared drain as well as the lateral drains (red). You are only responsible for the drain on your property that is not shared (purple).
How do you know if a drain is shared?
The best way to find out if your property is connected to the public sewer system is through a drain survey. Professionals can look at the system in place, contact the local council and water suppliers, and inspect sewer maps to establish which drains are shared and which drains are your responsibility.