A 58-year-old woman presents with sudden onset pleuritic chest pain and dyspnoea. She returned from a long-haul flight 3 days ago. Heart rate is 110 beats/min, respiratory rate 24 breaths/min, blood pressure 120/74 mmHg, oxygen saturation 94% on air and temperature 37.4°C. Her Wells score for pulmonary embolism is 5 (PE likely). What is the most appropriate next investigation?