Think this may be down to my host provider (Linode) restricting SMTP ports. Have requested unrestricting and will see if that helps as I have other apps with SMTP issues currently. This would be a good thing to include in the self host documentation as it's not obvious the ports need to be open when using an external mail provider 😊 Will report back with the result...

Confirmed & Solved - Need to unrestrict SMTP ports on your host provider to be able to send emails from Cal.com self hosted - even when using a mail provider. Unfortunately, this took me hours to establish and could have easily been avoided if the documentation had been clearer.