A 28-year-old woman presents with sudden onset shortness of breath and pleuritic chest pain 10 days after a long-haul flight. Her heart rate is 110 bpm, respiratory rate 24/min, BP 120/80 mmHg, and oxygen saturation 94% on air. Her Wells score is calculated as 5 (HR >100, recent travel >4hrs, clinical signs of DVT are absent but PE is the most likely diagnosis). What is the most appropriate next investigation?