Thought I'd post this just in case anyone else runs into the same problem.
The script truncates cPanel passwords at 20 characters, if like me you use long passwords, you will need either to edit the installer or install then edit the mysql row. Please note that when you go to install, it will create the admin user (because it's using your form data) but when new users try to register, it uses the truncated password and therefore is unable to create them (resulting in it attempting to load for a fair while 10-20 seconds before coming up with an empty page. Excellent script by the way, very useful. Thanks for putting the time and effort in to create it, you have my gratitude.