Ok, so I've used that script and made the zimbra-init file. It's stored in the right place and I've chmodded it to 755 to match the rest of the scripts in there.
I used chkconfig zimbra-init on to make it run on start and that should be it.
I've tried just executing it from the command line with ./zimbra-init but it errors on the line . /etc/rc.d/init.d/functions
I've not rebooted the machine yet as I'm working remotely and I'd rather perform the reboot in a few hours when I can be standing infront of it!