Patient Pathway through the Study: Once the patient consents to taking part on the trial the patient will enter the screening period. This can last from 1 to 14 days before the proposed start of the study treatment. As part of the screening period the patient will be required to attend a clinic where a doctor will complete a physical examination, assess the patients' vital signs and performance status, measure the patients' weight and collect information on the patients' medical history and any medications the patient is currently taking. Patients will also be asked to undertake routine blood tests and a MRI/PET-CT scan; if these have been completed as part of routine care; prior to consent and within the screening time lines; the results can be used for the study. Female patients will be asked to complete a pregnancy test up to 72 hours before confirmation of study eligibility. If the patient is confirmed as eligible then the patient will be receive a preloading dose of pembrolizumab (at the dosage currently under evaluation) 2 weeks prior to the start of chemoradiotherapy and then every 3 weeks until the patients have completed all 7 cycles, the patient disease progresses, the patient suffers from too many side effects or the patient decides they no longer wish to take part in the study. Whilst on treatment at each clinic visit before the drug is given the patient will also be required to attend a clinic so the doctor can complete a physical examination, assess the patients' vital signs and performance status, measure the patients' weight and collect information on any side effects the patient may have and any medications the patient is taking. At this visit the patient may also be required to give blood for routine blood tests and research blood samples if the patient have consented to do this. Once all the above assessments have been verified by the doctor Pembrolizumab will be given by a drip into the patients' arm that will take approximately 30 minutes. Patients will also be asked to attend a visit for a MRI/PET-CT at pre-brachytherapy (week 5), after the last dose of pembrolizumab (week 18) and then every 3 months for 2 years or until disease progression, discontinuation or initiation of another anticancer treatment. This is to see what is happening to the cancer as the patient continues on the study. FollowUp: When a patient completes their last dose of pembrolizumab the patient will be asked to attend post treatment follow up visit 4 weeks after their last dose of pembrolizumab and a safety follow up at 19 weeks from their last dose or before the initiation of a new anti-cancer therapy; whichever comes first. At the post treatment follow up visit and SFU visit the doctor will complete a physical examination, assess the patient vital signs and performance status, measure the patient weight and collect information on the patients' cancer, any side effects the patient may have and any medications the patient is taking. Patients will also have routine bloods taken. If the patient has completed all 8 doses and has not progressed at the safety follow-up (SFU) the patient will be reviewed for tumour progression and initiation of new anti-cancer treatments every 12 weeks for 2 years to review the disease status and if the patients have received any additional anticancer treatments until progression, initiation of a new anti-cancer therapy, withdrawal or completion of the study.