Visit two (4-6 days after visit 1, research facility) Participants will be asked to attend for a morning visit having omitted their morning insulin and not consumed anything (except water) from midnight the night before. If they feel unwell, or need to treat low blood glucose the visit will be postponed. A small cannula will be inserted to enable multiple blood samples to be taken with the minimum of discomfort. A fasting blood test (approximately 15mls) will be performed for glucose, c-peptide, glucagon and saved serum/plasma. Participants will be asked to take their normal morning insulin with a 10% reduction immediately prior to consuming 250mls of a meal replacement drink (Fortisip) with or without a DPPIV inhibitor (Vildagliptin 50mg) tablet. Blood for glucose, c-peptide, glucagon and saved serum/plasma will be taken at 30, 60, 90, 120, 150 and 180 minutes after they have completed the drink (volume of blood at each time point approximately 15 mls). A urine sample will be collected and saved at 180 minutes, and then the CGM device will be disconnected and removed. Participant's bedside blood glucose will be measured, and a light lunch provided together with advice from study clinicians regarding insulin adjustments if required.