I think that I've just about cracked it. Rather than renaming the existing file, I'm just deleting it before the newer file is downloaded. The download is set every hour and WC 'grabs' the image from the folder every 30 minutes (although this could be set to one hour too as there is only going to be a nano-second when there isn't a jpg file to be read by WC.
No upload to anywhere so no FTP needed, just viewable in in the WC Imagery window with animation also being available. I probably wouldn't incur the wrath of the Dundee Receiving Station if I did upload the odd image to my website, but if I decide to do that I'll check with them first. The images are provided free, but registration is required to view them.
So, the script:
1. Deletes any .jpg images in my /Desktop/Weather_Sat/ folder
2. Downlads the latest full-disk image from the Dundee Satellite Receiving Station, passing the username and password with the URL
3. Waits 3600 seconds before doing it over again.
Weathercat > Preferences > Imagery has the location of the file in /Desktop/Weather_Sat/ and displays it in the Imagery Window.
That's it, that's all I wanted to do. The only complication was the requirement to send a username and password otherwise I wouldn't need the script at all
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on idle
tell application "Finder"
delete (every item of folder ("Weather_Sat") whose name ends with ".jpg")
end tell
do shell script ("curl
http://username:password@www.sat.dundee.ac.uk/xrit/latest/latest_MSG_4_S4_grid.jpg >> ~/Desktop/Weather_Sat/latest_MSG_4_S4_grid.jpg ")
return 3600
end idle