Inclusion Criteria:  Histologically confirmed, newly diagnosed de-novo supratentorial glioblastoma (including gliosarcoma) Age ≥18 years Tumour deemed appropriate for surgical debulking Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 Clinically fit for, and appropriate to receive, neoadjuvant ipilimumab followed by standard of care treatment, based on investigator and MDT judgement Adequate organ and bone marrow function: Hb ≥9 g/dL, neutrophils ≥1.0 x 10 9/L, platelets ≥100 x 10 9/L and lymphocyte count ≥1.0 x 10 9/L Adequate renal function: < 1.5 x ULN or a creatinine clearance of ≥ 50mL/min calculated by Cockroft-Gault equation  Adequate liver function, including:  Bilirubin ≤ 1.5 x ULN (except for patients with known Gilbert's Syndrome who may have total bilirubin ≤ 3 x ULN) Aspartate or alanine transferase (AST or ALT) ≤ 2.5 x ULN Life expectancy of greater than 12 weeks Willing to comply with the contraceptive requirements of the trial Willing and able to comply with scheduled visits, treatment plan, laboratory tests and other study procedures Willing to donate tumour material and serial blood samples Written informed consent  Exclusion Criteria:  Diagnosis of Multifocal glioblastoma (Multicentric glioblastoma permitted) Prior resection of glioblastoma leaving inadequate tissue for post investigational treatment resection Secondary glioblastoma (i.e. previous histological or radiological diagnosis of lower grade glioma) Known extracranial metastatic or leptomeningeal disease Prior treatment for glioblastoma other than a limited resection or biopsy Dexamethasone dose >3mg daily (or equivalent) at the time of starting study treatment Antibiotics within 30 days of starting study treatment Intratumoural or peritumoural haemorrhage deemed significant by the treating physician  Active autoimmune disease apart from:  Skin conditions such as psoriasis, vitiligo or alopecia not requiring systemic treatment Type 1 diabetes or thyroid disease, controlled on medication Any evidence of severe or uncontrolled diseases (e.g. unstable or uncompensated respiratory, cardiac, hepatic or renal disease) Known hypersensitivity to ipilimumab or any of its excipients Past medical history of interstitial lung disease, idiopathic pulmonary fibrosis, drug-induced interstitial disease which required steroid treatment or any evidence of clinically active interstitial lung disease Any condition requiring systemic treatment with corticosteroids (>10mg prednisolone daily or equivalent) or other immunosuppressive medications within 14 days of starting study treatment. Inhaled or topical steroids, and adrenal replacement steroid doses > 10mg daily prednisolone or equivalent are permitted in the absence of active autoimmune disease Treatment with any other investigational agent within 28 days prior to starting study treatment History of previous cancer within 5 years, with the exception of adequately treated cone-biopsied in situ carcinoma of the cervix uteri and non-melanoma skin lesions Positive serology for Hepatitis B defined as a positive test for HepB surface antigen (HBsAg). Note: patients who are HepB core antibody (HBcAb) positive will only be eligible for the study if the HepB virus deoxyribonucleic acid (DNA) test is negative and patients are willing to undergo monthly monitoring for Hepatitis B virus reactivation Positive serology for Hepatitis C defined as a positive test for Hepatitis C virus antibody Diagnosis of prior immunodeficiency or organ-transplant requiring immunosuppressive therapy or known HIV or acquired immunodeficiency syndrome (AIDS)-related illness History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, or is not in the best interest of the subject to participate, in the opinion of the treating investigator Women who are pregnant or breast feeding