Incase you have found this post while searching for a solution to a problem with fail2ban - here is my final post and a possible solution.
Falco was correct - After a weeks worth of research I have concluded that it is difficult to customize the apache error log. You can easily customise the apache access log but unfortunatly not the error log.
The good news is that I was barking up the wrong tree.
fail2ban is a log parser and in the:
folder there are a number of regular expressions that you can modify to suit your logfile.
The answer to my problem was in the file.
Good luck and I hope this helps.