Thanks for your response. The hosting I'm using offers hybrid Windows/Linux hosting (however that works). Don't worry though - I'm not using that. They do have the option for .htaccess though and enabling it creates an .htaccess file in the root (outside of and same level as public_html). I've disabled that though.
The hosting has told me default docs don't matter for this, while the developer told me I should have one (though I'm pretty sure now they meant to say "shouldn't"). So I've removed my entry from the default docs list, enabled 'wildcard mapping to ASP .NET' and set a dedicated application pool for the site. Result now is an error page:
Server Error in '/' Application.
Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.
Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="Off"/>
</system.web>
</configuration>
Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>
Developer told me I'd just upload the files and it'll just work.