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The solution is to download the file to your computer and run it from there (rather than watching it in your browser). When it opens in your browser it doesn't have an immediately apparent save button. But I discovered the solution to this by accident. At the top-right is a button "Exit"-Home. Click this and a menu appears along the top. Click on the Floppy, as usual, to save.
Many thanks to Kevin Crowley at the CAA