The Investigators are researching how to improve the treatment currently available for patients diagnosed with Pseudomyxoma Peritonei (PMP). This is a rare cancer that usually starts in the appendix and spreads around the abdomen. PMP is usually treated using a type of surgery called Cytoreductive Surgery (CRS). During the surgery heated chemotherapy will also be used to treat any cancer cells that cannot be seen and may be left behind. This is called Hyperthermic Intraperitoneal Chemotherapy (HIPEC). This treatment is commonly used in the UK and in Europe, however, the chemotherapy can be given at two different doses: a lower dose over 60 minutes or a higher-dose over 90 minutes. The Investigators want to understand if there is a difference between these two doses. The higher dose has been associated with a slightly increased rate of complications but may be better at killing cancer cells and preventing recurrence of cancer. In Basingstoke the lower dose over 60 minutes is used and survival results are similar to centres who use the higher dose. Previous studies have shown that both doses are effective at treating PMP, but no research has shown which is better for patients. The Investigators hope to show that the lower-dose over 60-minutes is as good as the higher-dose over 90-minutes.