Inclusion Criteria:  Newly diagnosed or recurrence of clinical stage III-IVb histologically confirmed oral cavity, larynx, hypopharynx or oropharynx squamous cell carcinoma of the head and neck (SCCHN) Disease is considered resectable, definitive surgery is planned in the next 8 weeks from screening, and the patient is willing to undergo surgery Known human papillomavirus (HPV) status for oropharyngeal cancer Provide written informed consent to participate Aged 18 years or over Willing to consent to tumour biopsies at baseline Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 Ability to comply with study procedures in the Investigator's opinion Adequate renal function Adequate hepatic function Adequate bone marrow function Coagulation profile within the normal range Meeting the reproductive status requirements of the study  Exclusion Criteria:  Prior allogeneic or autologous bone marrow or organ transplantation Active infections requiring antibiotics, physician monitoring or recurrent fevers (>38.0˚C) associated with a clinical diagnosis of active infection. Active infection requiring systemic therapy within 1 week of the anticipated first dose of study drug. Active viral disease or positive test for hepatitis B virus using hepatitis B surface antigen test or positive test for hepatitis C virus (HCV) using HCV ribonucleic acid (RNA) or HCV antibody test indicating acute or chronic infection. Positive test for HIV or AIDS Patients who have active autoimmune disease that has required systemic therapy in the past 2 years, are immunocompromised in the opinion of the Investigator, or are receiving systemic immunosuppressive treatment Treatment with any COVID-19 vaccine in the 28 days before the first dose of NG-641, unless the vaccine is known to not be based on an adenoviral vector (e.g. messenger RNA (mRNA) vaccines) Treatment with any vaccine (including known non-adenoviral COVID-19 vaccines) in the 7 days before first dose of NG-641 History of clinically significant chronic liver disease History of clinically significant interstitial lung disease (including pneumonitis) History of prior Grade 3-4 acute kidney injury or other clinically significant renal impairment Use of the following antiviral agents: ribavirin, adefovir, lamivudine or cidofovir within 7 days prior to the first dose of study treatment; or pegylated interferon (PEG-IFN) in the 14 days before the first dose of study treatment Incomplete recovery from surgery, incomplete healing of an incision site or evidence of infection Any of the following in the 3 months before the first dose of study treatment: Grade 3 or 4 gastrointestinal bleeding or risk factors for gastrointestinal bleeding, infectious or inflammatory bowel disease, pulmonary embolism or other uncontrolled thromboembolic event, history or evidence of haemoptysis, or significant cardiovascular or cerebrovascular event Any known coagulopathy Prior history of bowel obstruction, or infectious or inflammatory bowel disease in the 3 months before the first dose of study treatment Major surgery or treatment with any chemotherapy, radiation therapy, biologics for cancer or investigational drug/therapy in the 28 days before the first dose of study treatment Other prior malignancy active within the previous 3 years, except for local or organ confined early stage cancer that has been definitively treated with curative intent, does not require ongoing treatment, has no evidence of residual disease and has a negligible risk of recurrence and is therefore unlikely to interfere with the primary and secondary endpoints of the study, including response rate and safety Tumour location/extent considered by the Investigator to present a significant risk of airway obstruction if tumour flare or necrosis were to occur (e.g. an initial increase in tumour size that may lead to intestinal, airway or ureter obstruction, or penetrating tumour infiltration of major blood vessels, or other hollow organs potentially at risk of perforation) Any serious or uncontrolled medical disorder that, in the opinion of the Investigator or the Medical Monitor, may increase the risk associated with study participation or study treatment administration, impair the ability of the patient to receive protocol therapy or interfere with the interpretation of study results Previous treatment with any other enadenotucirev-based therapy, or fibroblast activation protein (FAP) targeting agent Known allergy/immune-related adverse reactions to NG-641 transgene or immune checkpoint inhibitor products or formulation; severe hypersensitivity to another monoclonal antibody Any other medical or psychological condition that would affect the patient's ability to comply with all visits and assessments, or compromise ability to give informed consent Related to or a dependent of the site staff, or a member of the site staff