A 70-year-old man with a history of heart failure (LVEF 35%), hypertension, and chronic kidney disease (eGFR 40 mL/min/1.73m²) presents for medication review. He is currently taking Ramipril 5mg OD, Bisoprolol 5mg OD, and Furosemide 40mg OD. Despite this, he remains symptomatic with mild ankle oedema and NYHA class II symptoms. What single medication addition would be most appropriate according to NICE guidelines?