17. A detecting data processing apparatus operable to determine at least one data word from code words recovered from a marked material item, the at least one data word including at least two fields, values of a first of the at least two fields being associated with a first set of code words, values of a second of the at least two fields being associated wit a second set of code words, the second set of code words being selected in a dependence upon a value of the first of the at least two fields, the detecting apparatus comprising: a code word re-generator operable to re-generate code words each having a plurality of code word coefficients and being independently generated from a different code word seed; a correlator operable to correlate the re-generated code words with the recovered code words; and a control processor operable to control the code word re-generator to re-generate selectively each first code word associated with a possible value of the first of the at least two fields, to arrange for re-generated first code words to be correlated with the recovered code words in a first correlation, to identify, from a result of the first correlation, the value of the first of the at least two fields, to re-generate selectively each second code word associated with a possible value of the second of the at least two fields and selected in a dependence upon the value of the first of the at least two fields, to arrange for re-generated second code words to be correlated with the recovered code words in a second correlation, and to identify, from a result of the second correlation, a value of the second of the at least two fields.
19. The detecting data processing apparatus as claimed in claim 17 , wherein each of the code words is generated from a seed, seeds for each possible code word of the code words being stored in a data store, the code word re-generator being arranged to generate the code words in accordance with keys, the plurality of code word coefficients being pseudo-random numbers.