Multisite has been hacked

Training looks fine, business shows a blank page.

Your root page https://entapeople.org.uk/ shows a dialog from your hosting provider asking for uploads.

How have you configured your multisite? I’m more inclined to think this is an issue with the configuration rather than WP or your theme itself. For example, the links to “training” and “business” in your /home site, points to google.com instead of your other sites.