The problem there is I get a critical error when I try to access the file manager;

Been like that since early 6.0.x  (or late 5.x) if I recall and never been fixed with any update. At the time when I did the early 6/late 5 update and noticed it was broken I saw other people post about the file manager breaking for them too so figured it would be fixed in a update. I rarely use the DNN file manager as I use a FTP client to do any file access I need too. In fact I didn't realize it was still broken until I just tried to use it now per your request.

Is there something you want me to ask my ISP for specifically on that folder? Do you want just a list of the current permissions?

From doing a little searching it looks like your web.config file might have some issues and that is causing the issue with the File Manager. Can you see if you have the following info in your web.config file?

<folder defaultProvider="StandardFolderProvider">
      <providers>
        <clear />
        <add name="StandardFolderProvider" type="DotNetNuke.Services.FileSystem.StandardFolderProvider, DotNetNuke" />
        <add name="SecureFolderProvider" type="DotNetNuke.Services.FileSystem.SecureFolderProvider, DotNetNuke" />
        <add name="DatabaseFolderProvider" type="DotNetNuke.Services.FileSystem.DatabaseFolderProvider, DotNetNuke" />
      </providers>
    </folder>

<section name="folder" requirePermission="false" type="DotNetNuke.Framework.Providers.ProviderConfigurationHandler, DotNetNuke" />