1. A method of comparing a first document and a second document, wherein the text of each of the first and second documents can be divided up into basic units, the method comprising: generating a first sequence of images of basic units from the first document; generating a second sequence of images of basic units from the second document; comparing the basic unit images of the first sequence with the basic unit images of the second sequence to identify matches between the basic unit images of the first sequence and the basic unit images of the second sequence and hence differences identifying basic units of the first and/or second sequence that do not match and/or for which the difference between matched basic units exceeds a predetermined threshold; and outputting information about the differences between the first document and the second document.