I think I have it!
In the Control Panel/FTP & Connections/Connections tab, I had “Use Weather Display’s own FTP” unchecked. This is because I run a local server and the files don’t have to be uploaded by FTP. However, I have re-selected the checkbox for this option and now the file is being uploaded properly, at the scheduled interval.
Thanks to everyone who helped. You don’t know how much this means to me. It’s amazing how much you forget after having not had to touch the settings after so long.
Very grateful to you all! I don’t know what I’d do without you.
Cheers,
Dan