A 58-year-old man presents with a 3-month history of exertional chest tightness, which is relieved by rest. The pain does not radiate and lasts for about 5 minutes. He has a history of hypertension and smokes 20 cigarettes a day. An exercise tolerance test (ETT) shows 1mm ST depression in leads V4-V6 at peak exercise. According to NICE CG95, what is the most appropriate next diagnostic investigation to confirm coronary artery disease?