A 68-year-old man taking gliclazide modified release for type 2 diabetes is found drowsy with a blood glucose concentration of 2.1 mmol/L. He improves after intravenous dextrose, but his glucose falls to 2.8 mmol/L twice during the next four hours despite further dextrose. Renal impairment is noted and he cannot safely swallow. Which one of the following is the most appropriate treatment to prevent further hypoglycaemia?