A 55-year-old man with known stable angina presents for review. He is taking aspirin 75mg OD, atorvastatin 40mg ON, and uses his GTN spray occasionally. His symptoms are generally well-controlled, but he reports exertional breathlessness limiting strenuous activity. His heart rate is 75 bpm and BP is 130/80 mmHg. What is the most appropriate next step in his pharmacological management?