A 52-year-old man with dilated cardiomyopathy (ejection fraction 28%) and NYHA Class III heart failure presents with recurrent episodes of palpitations. A 24-hour Holter monitor reveals runs of non-sustained ventricular tachycardia (NSVT) lasting 5–8 beats at a rate of 160 bpm, occurring 3–4 times per hour. His blood pressure is 110/70 mmHg, and he is haemodynamically stable. Electrolytes are normal. Coronary angiography performed 6 months ago showed no significant stenosis. What is the most appropriate next step in management?