I think I'm sorted now, looks like I was asking the wrong question
I had already tried clicking on the index file and copying the URL from the browser for my URL link, but that wasn't working. After further research I found that I did not have slash arguments turned on in the Server > HTTP Configuration. Once I'd turned that on, I could grab a link to a legacy course file index page, and it worked fine. Plus turning on this setting is necessary to get links working within a folder using the "set main file" method, so it was an important find for me!