A 48-year-old woman with known primary adrenal insufficiency on maintenance Hydrocortisone presents with vomiting, abdominal pain and profound weakness after a flu-like illness. Observations: blood pressure 80/50 mmHg, sodium 124 mmol per litre, potassium 5.8 mmol per litre, glucose 3.2 mmol per litre. What is the single most appropriate immediate treatment?