Extensive transfer pricing documentation guidance has been released by the UK tax authority, clarifying many matters hitherto unspecified by them. These include (i) the introduction of a materiality threshold for documentation; (ii) guidance on the extent to which aggregation of transactions is acceptable; and (iii) the obligation to update data on comparables every year. Although submitting a Master file and Local File is only compulsory for the largest groups, the guidance states a strong expectation that all taxpayers should use this format in the future.