@louis, this was a great comment thanks. I think my issue was not adding /usr/bin/check_mk_agent to the command line to execute.
I have also used the actual IP address instead of $HOSTNAME$, not sure if that made any difference.
By the way what do you get when executing check_mk_agent | head on the HA side?
<<<check_mk>>>
Version: 2.1.0p20
AgentOS: linux
Hostname: a0d7b954-ssh
AgentDirectory: /etc/check_mk
DataDirectory: /var/lib/check_mk_agent
SpoolDirectory: /var/lib/check_mk_agent/spool
PluginsDirectory: /usr/lib/check_mk_agent/plugins
LocalDirectory: /usr/lib/check_mk_agent/local
FailedPythonReason:
I am a bit concerned about that FailedPythonReason:.