A 62-year-old man with chronic systolic heart failure (LVEF 35%) due to ischaemic cardiomyopathy is clinically stable on frusemide alone. He has no contraindications to other medicines. His blood pressure is 125/75 mmHg and creatinine is 80 µmol/L with normal potassium. What is the most appropriate next step to optimise his long-term management?