Alt+f then Alt+s
Menu commands use Alt rather than Ctrl.
This is windows standard.
Our app underlines the shortcut keys as you press Alt.
Andy, I hope you won't be offensed by my insistency, but have a look at any Windows program (Word, Excel, PowerPoint, Notepad, Wordpad, Paint, ...) : all of them have a Ctrl-S shortcut allowing direct save without going through the menu.
I agree, you have the same result with Alt-f + Alt-s and it will work perfectly fine. It's just that I think Windows users are used to the Ctrl-S shortcut the same way everybody is used to Ctrl-C and Ctrl-V to copy and paste.
Note on the attached screenshot: "Enregistrer" means "Save" in French.