Hi Dougall,
Thank you for reaching out to us, The demo content is only available in the X theme. Pro theme does not have that feature. In X, Installing demo content will not alter any of your pages or posts, but it will overwrite your Customizer settings. This is not reversible unless you have previously made a backup of your settings by going to the X > Validation menu Theme Options Manager section:
In standard demos, the importer also intelligently discerns which items you do and do not have present, so nothing will be mindlessly duplicated or repeated if you go through many demos, keeping your content as clean as possible. So you can change demos anytime but always take a backup of your settings.
You can also see https://theme.co/apex/forum/t/setup-demo-content/56
Hope this helps!