Inclusion Criteria: SCREENING/PRE-CONSENT: 1. Female and male patients with refractory focal epilepsy 2. Aged ≥ 18 years (no upper age limit but deemed medically fit for surgery with a life expectancy of at least 5 years) 3. Patient lives within 1 hour of transfer to an acute neurosurgical unit 4. Being considered for resective brain surgery (to remove the epileptogenic focus) based on first-stage preoperative assessments carried out as part of routine clinical care within 2 years of registration, showing: 4.1. Absence of vascular brain lesions or vascular malformations and/or cancer in the resection area (as confirmed on MRI) 4.2. Absence of active, untreated psychiatric disease in the opinion of the treating clinician (as confirmed by neuropsychiatric assessment) 4.3. Patient requires second-stage intracranial EEG investigations to be carried out via burr hole surgery to further assess eligibility for resective brain surgery PRE-REGISTRATION: 5. Patient deemed clinically suitable for resective brain surgery (i.e. a single region of seizure onset in the neocortex has been identified, and it does not overlap with areas necessary for critical functions such as language), as confirmed by intracranial EEG investigations 6. Patients who are women of childbearing potential (WOCBP), or male patients with female partners who are WOCBP or pregnant must agree to use highly effective methods of contraception from the time consent is signed until three months after treatment. Men (if applicable), must also advise their female partners regarding contraceptive requirements as listed for female patients who are WOCBP or pregnant. 7. Able and willing to give written informed consent to join trial Exclusion Criteria: 1. Not deemed clinically suitable for resective brain surgery (e.g. because of failure to identify a single region of seizure onset in the neocortex, region is too extensive or the region overlaps with areas necessary for critical functions such as language), as confirmed by intracranial EEG investigations 2. Vascular brain lesions or vascular malformations in area of planned resection 3. Detection of active cancer or on systemic treatment for cancer 4. Known or suspected HIV infection (confirmed by PCR test) and/or taking antiretroviral therapy 5. Patient deemed medically unfit for anaesthesia and surgery 6. Active, untreated psychiatric disease in the opinion of the treating clinician 7. Concurrent and/or recent involvement in another clinical trial of an investigational medicinal product (within last 3 months) 8. Females who are pregnant (confirmed by serum/urine ß-HCG) or actively breast-feeding 9. Known allergies to excipients of lentiviral gene therapy 10. Patient unlikely to cooperate with a 5-year follow-up; medical or psychological condition at the discretion of the investigator which would not permit compliance with the protocol or meaningful written informed consent 11. Any other known condition which is assessed as an intolerable risk by the investigator upon inclusion in the trial