Who is responsible when renting a house in the UK for plumbing problems?
You would be responsible for any incidental unblocking of a waste pipes (basin or sink) if your WC were blocked by say toilet roll tubes, sanitary items or misuse then you could be recharged. However, long term problems would be the Landlords responsibility; check your Terms and Conditions of Tenancy. If your septic tank is full and need emptying, that is the Landlords Responsibility and can become a Public Health problem, in which case your Landlord would be dealt with severely in court by the Local Authority. Your Local Water Board would be happy to explain that to your Landlord.