This study is a clinical trial in patients with Parkinson's disease, of a drug called Exenatide which is already licensed for the treatment of patients with Type 2 Diabetes. There have been several groups that have confirmed that Exenatide has beneficial effects on nerve cells when tested in the laboratory, that raises the possibility that Exenatide may slow down or stop the degenerative process of Parkinson's disease. In an open label trial in patients with Parkinson's disease who self administered the drug for 1 year, we have previously shown that the drug is well tolerated and shows encouraging effects on the movement and non-movement aspects of the disease, even 2 months after patients stopped administering the drug. The next step is therefore to formally evaluate whether Exenatide really is a potential "neuroprotective" drug, i.e. stops the nerve cells dying in Parkinson's disease, by conducting a double blind, placebo controlled trial.