A 36-year-old woman presents to the emergency department with sudden onset pleuritic chest pain and dyspnoea. She is tachycardic (HR 112 beats/min), has a respiratory rate of 28 breaths/min, BP 118/70 mmHg and SpO₂ 92% on room air. She is on the combined oral contraceptive pill and recently returned from a long-haul flight. Her Wells score for pulmonary embolism is calculated at 7. What is the most appropriate next investigation?