A 70-year-old man with a history of ischaemic heart disease and heart failure (LVEF 35%) is found to be in atrial fibrillation with a heart rate of 130 bpm on a routine ECG. He is symptomatic with palpitations and mild breathlessness. His blood pressure is 120/80 mmHg. According to NICE NG196, which of the following is the most appropriate initial pharmacological agent for rate control?