I resolved it in the end.
From Command Line, I ran php /home/storm/sites/helpdesk-domain-co-u/public/api/cron.php
and it worked perfectly.
I could check that by going to Admin > Settings > System and changing Default Log Level to DEBUG. Then each cron shows in Dashboard > System logs as a "Cron Job" entry (not the "Auto Cron" entries which are caused by Agent activity)
Then on my hosting platform, I tried adding that as the command but it failed. Turned out that behind the scenes, the cron job was trying to run on php version 7.x.
I therefore had to force the php version in my command line to:
/usr/bin/php8.2 /home/storm/sites/helpdesk-domain-co-u/public/api/cron.php
I had an easy GUI to stipulate the frequency so all I needed was the command above. I hope that may help others in future.