A 72-year-old man with stable chronic heart failure (NYHA Class II) is reviewed in clinic. His current medications include ramipril 10mg OD, bisoprolol 5mg OD, and furosemide 40mg OD. His LVEF is 35%. Despite optimal doses of ACE inhibitor and beta-blocker, he remains symptomatic with mild breathlessness on exertion. His eGFR is 55 ml/min/1.73m² and potassium is 4.5 mmol/L. According to NICE NG106, which medication should be added next?