Good news, I found the cause of of the problem. I was using interface aliases to permit different postfix instances.
However these were configured in the traditional/legacy manner using /etc/network/interfaces. It seems that although the Postfix systemd unit employs the "network-online.target" dependency, this dependency does not recognise the legacy manner of configuring aliases. Therefore Postifx was crashing becuase it was unable to bind to the IPs that had not yet been bound to the interfaces. Once I re-configured networking in the modern style using systemd-networkd (/etc/systemd/network/if.network) Postfix was happy because the "network-online.target" dependency was now operating correctly.
