This trial aims to generate further data to explore whether 48 weeks exposure to Exenatide has an advantage over placebo based on a standard validated assessment of Parkinson's disease severity (the MDS UPDRS part 3 motor subscale). This will be measured during the "practically defined OFF medication state" i.e. after patients have withheld their conventional PD medication overnight. The hypothesis is that Exenatide will be associated with reduced MDS UPDRS part 3 scores at the study end.