Your question
What is my responsibility with regard to my wastewater pipes?
The waste water pipe connecting your property to the public sewer is called a drain.
If your property was built in 1937 or later the drain is your property and responsibility. Sections of drain serving more than one property are in shared ownership of those properties.
If your property was built before 1937 drains serving only your property are also your property and responsibility, but sections serving more than one property are owned and maintained by United Utilities.