Mesothelioma is a cancer that is caused by exposure to asbestos, an environmental contaminant. This cancer is incurable and lacks effective treatment, particularly after initial chemotherapy. There has not been a licenced therapy for mesothelioma since 2003, and no treatment has yet demonstrated an improvement in survival following initial chemotherapy. There is an urgent need to explore more effective approaches to therapy. Targeted treatments offer potential hope for the treatment of mesothelioma. A class of drugs called PARP (Poly Adenosine Diphosphate-ribose polymerase) inhibitors have already been proven to improve the survival of patients with breast and ovarian cancers, that carry specific mutations. Mesothelioma has been shown in a recent trial to respond to this class of agent. Further investigation is warranted to test whether PARP inhibitors could be a new treatment option for patients. As with ovarian cancer studies of the past, the NERO trial will test a PARP inhibitor (niraparib) after successful treatment with chemotherapy. Patients whose tumours shrink or stabilise following chemotherapy are expected to have a greater chance of benefit from niraparib. It's not known if niraparib will be able to improve survival of patients with mesothelioma, or indeed whether or not toxicity could occur without benefit. For that reason, patients will be randomised with a 2:1 chance of receiving the drug. Those patients who do not receive niraparib will be closely monitored for signs of early tumour growth so that they can go on to receive an alternative treatment if necessary. If the NERO trial is positive, this study will lead to the approval of a new medicine for use around the world, one that would extend the life expectancy of patients for the first time following initial chemotherapy. NERO will recruit 84 patients over 12 months. Those randomised to receive niraparib will receive a daily dose of 200 mg or 300mg for up to 24 weeks within the trial.