A 54-year-old man presents with five days of fever, dry cough, headache and diarrhoea after returning from a Mediterranean cruise. He is confused, respiratory rate is 32/min, blood pressure is 100/62 mmHg and oxygen saturation is 90% on room air. Chest radiography shows bilateral patchy consolidation; serum sodium is 124 mmol/L (135–145). He has no aspiration history, structural lung disease or recent antibiotic exposure. Which one of the following is the most appropriate initial antimicrobial regimen?