A 45-year-old woman presents to the Emergency Department with acute onset pleuritic chest pain and shortness of breath. Her observations are: HR 105 bpm, RR 22/min, SpO2 95% on room air, BP 120/80 mmHg. She has no significant past medical history but took a long-haul flight 5 days ago. Her Wells score for Pulmonary Embolism (PE) is calculated as 5 ('PE likely'). According to NICE guideline NG158, what is the most appropriate next diagnostic step?