OK, this is getting stranger.
I created a file named 'validator.php' in a directory under public_html on the remote server. I then made some changes and saved it as 'validator2.php' in the same directory.
Then tried the URL http://domain/path/validator2.php
and get a 404 Not found.
Tried re-saving the file in Studio and get a 'file already exists' message. I've checked the paths a dozen times. The file isn't on the remote filesytem and doesn't show up in the remote file browser in Studio. But it's there in the editor and Studio tells me it already exists if I 'Save As'.
So, wtf is the file - or where does Studio think it is?
Hair getting torn out here...