Hey Stuart,
When you say that you’ve downloaded some demo content, you mean actual demo content, or are you speaking of the XCS (X Customizer Settings) file that is used to import into the Customizer?
If you want to delete the demo content, you’ll need to go through manually and simply delete all pages, posts, media, et cetera.
If you want to revert your site to it’s original state, you can download the Integrity 1 XCS file and import that back into your site. This will revert everything like it is a brand new installation.
Thanks!