Re: Struggling to get dovecot working with postfix auth

That's a permission error. Somewhere in your directory hierarchy things are off. See Postfix' set-permissions command.

But surely if Dovecot is staring as root then directory permissions are relevant, especially if I'm then asking the config to chmod the file anway ?

To me, it seems dovecot is not behaving correctly, because if it is not using root to access the directory then it is not going to be able to chmod the socket later is it ?