Hi James,
I had a similar issue and I just found a solution, if you haven't already.
Here's how I solved it:
In my /etc/apache2/ folder I did a
grep -r "DirectoryIndex" .
I found I had
DirectoryIndex index.shtml
in my /etc/apache2/sites-available/default file.
After changing that line to DirectoryIndex index.html and reset the server it was working fine.
I hope it helps.