A 58-year-old man with cirrhosis due to chronic hepatitis C presents with increasing abdominal distension. Examination confirms shifting dullness consistent with ascites. His blood pressure is 110/68 mmHg, serum sodium 132 mmol/L and serum creatinine 85 µmol/L. There is no evidence of hepatic encephalopathy. What is the most appropriate initial pharmacological management of his ascites?