A 70-year-old woman with known stable angina, controlled with bisoprolol and GTN spray, presents for a medication review. She mentions that over the last few months, her angina symptoms have worsened, now occurring with minimal exertion (CCS Class III). Her heart rate is 55 bpm and BP is 110/70 mmHg. What is the most appropriate next step in her pharmacological management, according to NICE CG126?