Well with only 70% of my diskspace taken up I was getting out of space errors in zoneminder.
Turns out its my iNodes not the physical space on the disk.
pop this into crontab
0 0 * * * find /var/cache/zoneminder/events/ -name "*.jpg" -mtime +5 -exec rm -f {} \;
Change the +5 to be +howeverManyDaysYouWannaKeepYourImagesFor.
