Exenatide is a licensed, safe and effective treatment for patients with Diabetes mellitus. Laboratory work has shown strong, reproducible evidence that this drug has beneficial "disease modifying" effects when given to animals with a range of experimental models of Parkinson's disease (PD). This project aims to make an initial evaluation of possible benefits of Exenatide among patients with moderate symptoms of PD. The drug will be given as a twice daily 10microgram injection under the skin in a similar way to one of the conventional "symptomatic" treatments for PD (Apomorphine).