3. An encoding data processing apparatus for generating a marked copy of an item of material by introducing code words into a copy of the item, the code words being arranged to identify a data word from a set of data words having at least two fields, the apparatus comprising; a code word generator operable to generate at least two of the code words having a plurality of code word coefficients, each of the at least two of the code words being independently generated from a different code word seed, one of the at least two of the code words being selected from a first set in a dependence upon a value of a first of the at least two fields and another one of the at least two code words being selected from a second set in a dependence upon a value of a second of the at least two fields; and an encoding processor operable to combine the plurality of code word coefficients with the item, wherein values of the first of the at least two fields are associated with the first set and values of the second of the at least two fields are associated with the second set, the second set being selected in a dependence upon the value of the first of the at least two fields.
3. An encoding data processing apparatus for generating a marked copy of an item of material by introducing code words into a copy of the item, the code words being arranged to identify a data word from a set of data words having at least two fields, the apparatus comprising; a code word generator operable to generate at least two of the code words having a plurality of code word coefficients, each of the at least two of the code words being independently generated from a different code word seed, one of the at least two of the code words being selected from a first set in a dependence upon a value of a first of the at least two fields and another one of the at least two code words being selected from a second set in a dependence upon a value of a second of the at least two fields; and an encoding processor operable to combine the plurality of code word coefficients with the item, wherein values of the first of the at least two fields are associated with the first set and values of the second of the at least two fields are associated with the second set, the second set being selected in a dependence upon the value of the first of the at least two fields. 13. A web server operable to provide material items for downloading via the Internet, the web server including the encoding data processing apparatus according to claim 3 , wherein, the encoding data processing apparatus is operable to receive the material items, and the encoding data processing apparatus is operable to introduce a code word into the material items before the material items are downloaded.