As this is a clinical trial the participants will be randomly allocated to either allopurinol or a dummy medication (placebo) and will receive one year of treatment so that the investigators can compare if there is a difference between normal treatment and addition of allopurinol. All the patients currently prescribed medication for their high blood pressure will continue as normal on that. They will have a further MRI scan when their one year treatment with allopurinol or placebo finishes.