A 67-year-old man with long-standing hypertension and ischaemic heart disease presents with acute shortness of breath. He is orthopnoeic, tachypnoeic, hypertensive at 190/110 mmHg, with bibasal crackles and an S3 gallop. Oxygen saturation is 92% on room air. ECG shows sinus tachycardia with old Q waves. After sitting him upright and providing supplemental oxygen, what is the most appropriate immediate pharmacological therapy?