1. An apparatus comprising: at least one processor having an input and an output and at least one storage device connected for access by said at least one processor, apparatus being configured to convert a first data set comprising metadata structured according to one of a plurality of first predefined hierarchical metadata structures into a second data set comprising said metadata structured according to one of a plurality of second predefined hierarchical metadata structures, the apparatus being configured by executable computer program code to provide, during execution thereof: a store storing mappings, respectively corresponding to each of said first hierarchical metadata structures, between data elements of said first predefined hierarchical metadata structures and data elements of a predefined intermediate hierarchical metadata structure and mappings, respectively corresponding to each of said second predefined hierarchical metadata structures, between data elements of said predefined intermediate hierarchical metadata structure and data elements of said second predefined hierarchical metadata structures; an input which receives a data set comprising metadata to be converted; data conversion means arranged to select mappings from said stored mappings in dependence on structure of data elements in said received data set and in dependence on structure of a selected predefined target hierarchical metadata structure, and to apply the selected mappings to generate a target data set having said predefined target hierarchical metadata structure; and an output which outputs a converted data structure based on said target data set, on receipt of a first data set structured according to one of said plurality of first predefined hierarchical metadata structures, the data conversion means are configured to generate from said first data set structured according to said first predefined hierarchical metadata structure, a first target data set having said predefined intermediate hierarchical metadata structure and thereafter to generate from said first target data set having said predefined intermediate hierarchical metadata structure, a second target data set having said second predefined hierarchical metadata structure, wherein said first and second predefined hierarchical metadata structures are selected from a set of n predefined hierarchical metadata structures, each having hierarchical depth greater than 1, where n is an integer greater than 3; and wherein said store stores mappings between data elements of each of said n predefined hierarchical metadata structures, one of which is selected as said first predefined hierarchical metadata structure and data elements of said predefined intermediate hierarchical metadata structure, and between data elements of said predefined intermediate hierarchical metadata structure and data elements of each of said n predefined hierarchical metadata structures, one of which is selected as said second predefined hierarchical metadata structure.