I think I might‘ve gotten to the bottom of this. One of the other shares in the server is a TimeMachine location and it looks like smbd needs full disk access whenever you share a time machine backup destination. This seems to be linked to the above setting. I’d still like confirmation of this however.

I debugged this further and it turned out to be a permissions issue. The installer is looking for a "trust cache" which it can't access and you get a "permission denied" error when you run it as a standard user. If you run the installer using sudo in an admin account it works fine! I've never encountered this before and given very little actually changes on this machine, I've no idea what could have caused this.

Thanks for getting back to me! Yes I'm inclined to agree - as much as I hate to have to do it I suspect I'll end up having to re-install. What's frustrating with this is the fact that I've not really changed much on this machine and without me knowing what went wrong I can't be certain this won't happen again in the future :) Plus when I set this up a while back I made a fair few customizations and I'm not sure if a TimeMachine restore will get it back to the way it was. Probably will just have to take the hit. The developer in me wants to keep digging through logs to see if I can debug further so I might do a bit more following your guide you linked. Cheers.