Parkinson's disease (PD) is a devastating neurological condition. It affects over 10million people worldwide and UK data shows that the number of people affected will increase by 28% by 2020. The most obvious symptoms are altered motor functioning, although many patients experience non-motor symptoms. Up to half experience psychosis during the course of the disease. Visual hallucinations (seeing things that are not there) and delusions can be extremely distressing resulting in poorer quality of life and increased caregiver burden. Existing treatments for Parkinson's disease replace lost dopamine (a brain chemical) in the brain, but do not help with psychosis symptoms. Existing treatments for psychosis (as would be given to patients with schizophrenia) are ineffective or come with troubling side effects (sedation, heart and liver function) and regular monitoring visits. Brain imaging studies have shown that another brain chemical, serotonin, is involved in psychosis symptoms. We have gathered evidence to indicate that serotonin signalling through a particlar pathway in brain cells called the src-kinase pathway may be responsible for PD psychosis symptoms. If we can reduce activity in this pathway patients may see improvements in their symptoms and quality of life. A drug from cancer called saracatinib has been made available that reduces activity in the src-kinase pathway of the brain. Studies so far have shown that it can affect brain function and is well-tolerated. We wish to test this drug in patients with PD psychosis to show that this drug can reduce or normalise brain function in the brain areas that are associated with visual hallucinations and other disturbances. We will test the effects of 10 days of saracatinib against a dummy drug (or placebo) in 20 patients with PD psychosis and compare the effects to those without PD psychosis to see if it normalises brain function. We will use two brain measurements that are safe and well-tolerated for this study and ask patients to perform visual recognition tests or just rest while we take these measurements. If successful, this study will provide the basis to test the drug in larger samples which is necessary to develop a new treatment.