 The study will be conducted in two parts. PART A is a single centre, single dose design, open label. Patients with inoperable malignant pleural mesothelioma will receive a single dose of HSV1716 by intrapleural administration. Delivery will be by direct administration via an indwelling catheter into the pleural cavity. PART B is a single centre, repeat dose design, open label. Two groups of three patients with inoperable malignant pleural mesothelioma will receive 2 group 1 or 4 group 2 single doses of HSV1716 at weekly intervals. Administration will be via an indwelling catheter into the pleural cavity. 