Locally advanced cervix cancers (stage 1B-IV) are usually treated with radiotherapy, concomitant cisplatin chemotherapy and brachytherapy. Failure to achieve locoregional control (LRC) remains a problem, especially in the setting of stage III/IV disease. More importantly, however, the dominant unresolved problem remains the occurrence of distant metastatic relapse. With the knowledge that 99% of all cervix cancer is associated with human papillomavirus (HPV) infection, there is a strong rationale to consider immunomodulatory strategies in the radical management of this disease. Therefore, in this research protocol the investigator will treat patients with stage 1B-IVA carcinoma of the cervix planned to receive radical radiotherapy with concomitant cisplatin and brachytherapy. The research involves adding a new therapy in the form of an antiPD1 monoclonal antibody (pembrolizumab) to the standard treatment of radiotherapy combined with cisplatin chemotherapy and brachytherapy. This treatment seeks to activate the patient's own immune system to attack the cancer cells - and the investigator believes that adding this treatment during standard treatment may be particularly effective. Patients will receive an initial dose of pembrolizumab 2 weeks before starting a course of chemoradiotherapy and brachytherapy. In the first instance, patients will receive 100 mg of pembrolizumab and, if this is safe and tolerable in the first 3 patients, the dose will be increased to 200 mg for all other patients. Radiation will be delivered on 28 occasions with chemotherapy given intravenously in weeks 0, 1, 2 and 3. Brachytherapy will be given on 3 occasions after completion of the radiation. Additional doses of pembrolizumab will be given every 3 weeks for a further 7 doses. The investigator will assess the feasibility and safety of the combination of pembrolizumab with radiotherapy and cisplatin.