A 28-year-old man presents to the Emergency Department with a 1-hour history of sudden onset palpitations and light-headedness. His ECG shows a regular narrow complex tachycardia at a rate of 180 bpm. His blood pressure is 110/70 mmHg and he is otherwise stable. Vagal manoeuvres (Valsalva) have been unsuccessful. According to Resuscitation Council UK guidelines, what is the most appropriate next pharmacological intervention?