A 23-year-old man with type 1 diabetes presents with abdominal pain, vomiting and polyuria. He appears dehydrated and tachypnoeic. Capillary glucose is 28 mmol/L and capillary ketones are 6.0 mmol/L. Venous blood gas shows pH 7.15 and bicarbonate 10 mmol/L. According to UK guidance, what is the most appropriate initial intravenous fluid?