That’s right! I recently talked about these finer details with the most relevant aspect being:
If you’re running the latest subscription-manager on your CentOS 7 (or 8) hosts then katello-host-tools is not required at all unless you are using katello-agent instead of REX. (or Tracer)
All of this said, we’re aware of some issues around errata applicability:
One was fixed in 3.15.1: (Bug #2948: Whitelist additional Provisioning Template Macros - Foreman
Another fix is coming in 3.15.2 and later: Bug #29961: Actions::Katello::Host::UploadProfiles failing at upload_modules_to_pulp - Katello - Foreman.
I think @paulraines68 experiencing the former, so I recommend updating to 3.15.1 and checking. Let us know how it goes.