A 36-year-old woman presents with sudden onset pleuritic chest pain and dyspnoea. She is on the oral contraceptive pill and returned from a long-haul flight 3 days ago. Pulse is 110 beats/min, respiratory rate 24 breaths/min, oxygen saturation 96% on room air and blood pressure 118/70 mmHg. She has no signs of DVT. Using the Wells score, she is assessed as moderate risk for pulmonary embolism. What is the most appropriate next investigation?