A 32-year-old woman presents with sudden onset pleuritic chest pain and shortness of breath 2 weeks after a long-haul flight. She is tachycardic at 112 bpm, respiratory rate 26 breaths/min, oxygen saturation 95% on air and blood pressure 120/75 mmHg. Her Wells score suggests pulmonary embolism is likely. What is the most appropriate next investigation?