Thanks again Peter, I've got those new javascript files downloaded.
As far as I can tell the files are created on the fly when a preview is run, then deleted when the preview is closed. Every new preview creates a new folder; for example, my first preview was "E:\HelpCentre\!ScreenLayout!\Pegasus-Indigo4\!preview_folder_1!\template\scripts".
So, I don't see how I can stop the preview crashing happening.

But the files fixed the error after I built the project for real.
When the output is generated for real, the !SSL folder is updated, with the javascript files in the 'scripts' folder; for example, mine is E:\HelpCentre\!SSL!\Pegasus-indigo4\template\scripts. I copied the files here and that sorted it.
However, the scripts are updated each time the project is built so it looks to me that I need to copy the javascript files over after every build.
Is that right? Or is there another way?
Thanks very much for all your help.
You can always stick them in your "factory" install location in c:\program files...
Just be sure to rename the original ones to some non-functional named extension (like .old or .orig)