It took some time to get them properly in Google Webmaster Tools 0:)
It should ideally be across all pages of all sites, so as to declare all 'sibling' pages, e.g.
https://www.fortsu.co.uk/running-shoes/all/trail
https://www.fortsu.de/laufschuhe/alle/trail
https://www.fortsu.com/running-shoes/all/trail
you can also use XML sitemaps to implement it, which tends to be preferable, especially if you don't want the extra bloat in the HTML of your pages.
I will check the XML sitemap stuff, thanks for the tip.
It took some time to get them properly in Google Webmaster Tools 0:)