Method: The investigators will test the ability metformin, a drug with proven safety, to slow the progression of furring up (known as atherosclerosis) of blood vessels in non-diabetic subjects with heart disease. This will be achieved by treating 2 groups of subjects with metformin and placebo pills respectively. To measure atherosclerosis, the investigators will carry out ultrasound scans of the big blood vessels in the neck at the start of the study, after 1 year and after 1.5 years of therapy.The investigators will then be able to assess whether metformin has had a beneficial impact.