A 63-year-old woman with type 2 diabetes accidentally injects 60 units of insulin glargine instead of her usual 20 units. She becomes confused with a capillary blood glucose concentration of 2.3 mmol/L and is treated with intravenous glucose, with full recovery. Two hours later her glucose concentration has fallen again to 3.0 mmol/L despite eating a meal. Which one of the following is the most appropriate next step?