A 60-year-old man with ischaemic cardiomyopathy (LVEF 28%) remains NYHA class II despite optimal doses of an ACE inhibitor (ramipril), beta blocker (bisoprolol) and mineralocorticoid receptor antagonist (spironolactone). His blood pressure is 118/72 mm Hg, creatinine 95 µmol/L and potassium 4.7 mmol/L. According to CCS heart failure guidance, which pharmacologic change is the most appropriate next step to reduce mortality and hospitalisation?