A 24-year-old tall thin man develops sudden onset right-sided pleuritic chest pain and shortness of breath while at rest. He is haemodynamically stable. Chest X-ray shows a right-sided pneumothorax with a 3 cm rim of air between lung margin and chest wall at the level of the hilum. He has no underlying lung disease. According to BTS/NICE guidance, what is the most appropriate initial management?