Patients with type 2 diabetes have very variable endogenous insulin secretion. While some patients have relatively preserved endogenous insulin with marked insulin resistance others may develop the very severe insulin deficiency seen in type 1 diabetes. The impact of this variation on hypoglycaemia risk and treatment response in type 2 diabetes is unclear. This project aims to determine the impact of residual endogenous insulin secretion on glucose variability, hypoglycaemia risk and treatment response in insulin-treated participants with a clinical diagnosis of type 2 diabetes. The investigators will recruit participants from existing cohorts known to have severe insulin deficiency despite classical clinical characteristics of type 2 diabetes. The investigators will recruit other participants with insulin-treated type 2 diabetes and retained endogenous insulin secretion matched for glycemia and gender. The investigators will assess glucose variability (using continuous glucose monitoring system (CGMS)) and treatment response to a single dose of the glucose lowering therapy vildagliptin and compare responses between groups. This study will allow us to assess the potential utility of measuring endogenous insulin secretion in insulin-treated type 2 diabetes as a marker of hypoglycaemia risk and in determining likely response to oral therapy.