Neonatal diabetes is diagnosed before 6 months of age and causes high blood glucose levels due to the pancreas not secreting insulin. Neonatal diabetes can be caused by a change in a DNA region called the KCNJ11 gene. KCNJ11 encodes a channel in the pancreas that acts as a switch to turn 'on' and 'off' insulin secretion. A change in KCNJ11 results in a faulty channel, which keeps insulin secretion 'switched off'. The diabetes can be treated with tablets called sulphonylureas that switch the pancreatic channel 'on', allowing it to secrete insulin in response to gut hormones called incretins. Previous research has shown that patients who switch from insulin to sulphonylureas have better blood glucose control, including fewer episodes of hypoglycaemia (glucose dropping too low), and also avoid the need for injections. It is thought that serious side effects from sulphonylureas are uncommon in KCNJ11 neonatal diabetes. Some patients report low glucose after meals and we think this may be because they make too much insulin if they eat a meal with protein but low amounts of carbohydrate. The investigators will test this by giving study participants different meals and measuring the amount of insulin, glucose and incretin hormone in the blood afterwards.