Inclusion Criteria:  Clinical diagnosis of Type 1 diabetes Established on the Tandem pump for at least 1 month, using novorapid or humalog insulin HbA1c 40-90 mmol/mol inclusive within the previous 30 days or taken at screening Total daily dose of insulin 10-100 units Willing to attend study visits and complete all study assessments Able to give informed consent  Exclusion Criteria:  Pregnant or planning pregnancy Any disorder which, in the investigators' opinion might jeopardise the patient's safety or compliance with the protocol Life expectancy less than one year Severe renal impairment (eGFR <30ml/min) Weight <25kg Patients prescribed hydroxycarbamide