This is something that has just taken me ages to work out so I'm leaving myself a note!

To get custom error 404 pages to work on IIS using FCGI you need to prevent PHP from executing on folders, only files, as shown here. It's 1 option to change to get it working.



If you don't set this, you end up getting "Input file not specified" errors from PHP rather than a web server response.

Hopefully this helps others too.