I'm not quite sure what your input file looks like. It certainly seems to work OK if the dates are the first item on each line and are in the format:
which is what you need for a sort to work. Could you give us an example?
well the lines in the log start with
30-06-10_blah blah blah
01-07-10_blah blah <---- this is the culprit because this is put at the bottom of the log file, where as I want the latest to be always at the top. with sort -rn