That is not the case at all as illustrated by your username here. In addition to that the login is case insensitive. You can login with GraemeAstro, graemeastro or GrAeMeAsTrO.
Strongly suggest you delete the installation created through the hosting control panel and manually install it yourself.
If I rename the phpbb folder to phpbb_old would I then be able to do a fresh install properly using the same domain?
Just delete the folder, in the hosting control panel use phpmyadmin to delete the tables the panel install created. Presuming they are using default table prefix it will be phpbb_ That will completely remove the installation. When uploading the files downloaded here the phpBB3 folder can be named anything you want, forum for example. If you want to install in the root of the domain upload the contents of the phpBB3 folder.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison