1. An apparatus ( 400 ) for converting a data set structured according to a first predefined structure ( 405 ) into a data set structured according to a second predefined structure ( 410 ), the apparatus comprising: a store ( 420 ) for storing mappings between data elements of said first predefined structure ( 405 ) and data elements of a predefined intermediate structure ( 300 ) and between data elements of said predefined intermediate structure ( 300 ) and data elements of said second predefined structure ( 410 ); an input for receiving a data set to be converted; and data conversion means ( 440 ) arranged to select mappings from said stored mappings ( 420 ) according to the structure of data elements in a received data set and according to a predefined target structure ( 405 , 300 , 410 ), and to apply the selected mappings to generate a target data set having said predefined target structure ( 405 , 300 , 410 ), wherein, in use, on receipt of a data set structured according to said first predefined structure ( 405 ), the data conversion means ( 440 ) are operable to generate a target data set having said predefined intermediate structure ( 300 ) and, upon receipt of a data set structured according to said predefined intermediate structure ( 300 ), to generate a target data set having said second predefined structure ( 410 ).