The dataset is contained within an excel file. The file contains the data plotted in the figures in the corresponding paper. The study aimed to measure a record 962.5 % specularly reflected energy fraction from an interaction with a plasma mirror surface preionised by a controlled prepulse. Results determined that the optical quality is dependent on the inter pulse time delay. The files associated with this dataset are embargoed until 1 April 2015 and will be available here beyond this date.