A 62-year-old man with long-standing Addison's disease on hydrocortisone replacement presents to the emergency department with vomiting, diarrhoea, hypotension (BP 80/50 mmHg) and confusion after a viral illness. His skin is hyperpigmented; he is tachycardic and dehydrated. Capillary glucose is 3.4 mmol/L. What is the most appropriate immediate treatment?