1. A method of interpreting data in a first version of a data structure, the method comprising: determining if the first version of the data structure is different to a currently used version of the data structure; and if the versions are different, then for each field of the currently used version of the data structure, determining if a corresponding field is present in the first version of the data structure; and if not, adding data to the field in the currently used version of the data structure according to interpretation rules.