A 67-year-old man with known ischaemic heart disease and heart failure (LVEF 35%) presents with worsening shortness of breath and bilateral leg swelling. His current medications include Ramipril, Bisoprolol, Furosemide, and Aspirin. Examination reveals elevated JVP, bibasal crackles, and pitting oedema to mid-shin. According to NICE guidelines for chronic heart failure, which medication should be added next, provided his eGFR and potassium are satisfactory?