1. Data processing apparatus, comprising: data storage means having a plurality of defined storage elements, memory means containing usage data indicating which of said storage elements contains data, and processing means configured to update said usage data in response to data being stored within said data storage means, wherein said memory means further contains a datastore, and said processing means is configured to: analyse said usage data to determine the number of said storage elements not containing data; store information within said datastore indicating said number of storage elements not containing data; and read information from said data storage means to determine whether said further data may be stored.