1. A method for identifying updated data items in a plurality of data items stored at a data item storage system, the method comprising: receiving, at the data item storage system, data indicating a last synchronization time, wherein each data item in the plurality of data items has an associated update time, wherein identifying updated data items comprises identifying those data items in the plurality of data items that have an update time later than the last synchronization time, wherein the data items are updated such that each data item is associated with one of a plurality of update tokens, each update token being associated with updates performed up to an end of a respective bound period that commences at a time at which the update token is superseded by a successive update token, wherein information relating to each data item is stored using at least one of a plurality of files in a network file system, each file having a file name, file content and a last modified time that corresponds to the update time of said data item, wherein information relating to the update token of each data item is stored in the file name of at least one of the plurality of files storing the information relating to that data item, and wherein the update tokens are allocated successively in a monotonic sequence; identifying, at the data item storage system, an update token that is associated with one or more updates performed relative to a point in time related to the last synchronization time by the bound period; and identifying, at the data item storage system, data items having an update time after said last synchronization time based on said identified update token.