Water utilities (WU) in the UK are responsible for providing sewerage disposal for customers across the country. Blockages within sewer networks can be disruptive for customers, damaging to the local environment and costly to rectify. Effective scheduling of preventative maintenance (PM) is an important for WU to prevent blockages, reduce costs and protect the environment. In this paper, we describe a novel multi-objective optimisation methodology to the scheduling of PM applied to a case study in Swansea, Wales. The results of real-world trials demonstrate that solutions generated by the proposed method achieve a 13.8% increase in jobs completed for compared to the standard approach used in November 2019.