A 64-year-old woman presents with sudden onset dyspnoea and pleuritic chest pain. She returned from a long-haul flight 2 days ago. Oxygen saturation is 90% on room air, respiratory rate 28 breaths/min, heart rate 110 beats/min and blood pressure 112/70 mmHg. Wells score suggests moderate probability of pulmonary embolism (PE). What is the most appropriate next investigation?