A 68-year-old man with stable angina presents with increasing frequency of chest pain on exertion over the last month. His current medication includes aspirin 75mg OD, bisoprolol 5mg OD, atorvastatin 40mg ON, and GTN spray PRN. He is using his GTN spray 3-4 times a day. His heart rate is 60 bpm and BP is 125/75 mmHg. What is the most appropriate next step in his pharmacological management according to NICE CG126?