Thank you again Janis for taking the time to assist me. Your suggestion to purge the bacula packages (univention-remove --purge bacula*), while not completely solving my issue, certainly got me on the right track & I have now successfully installed a working univention-bacula set of packages. For the sake of clarity, & for anyone else who might find themselves in the same position, I'll explain below what solved the issue for me.
Running "univention-remove --purge bacula" removed all the bacula packages & suggested an "apt-get autoremove" to clean up dependencies no longer required (which I did also). Subsequent reinstallation of univention-bacula again failed to create the "bacula" postgresql database. My reference in an earlier post to a message during install "not updating univention/bacula/postgresql" was incorrect. The actual message was "not updating bacula/psql/database". (I am referring here to the messages displayed below the progress bar in the UMC Package Management module). I would be interested to find if/where those messages are logged to ?
So I then purged the bacula packages once more. I was reminded by your advice about dbconfig, & that postgresql is only required by the "software monitor" which I do not use. So I then ran a purge of the dbconfig & postgresql packages. Funnily enough, during this process, dpkg seemed to think that a number of univention-bacula-enterprise packages required removal, then gave an error those packages could not be found when it attempted to remove them.
In any event, subsequent re-installation of univention-bacula from "Package Management" in the UMC installed all the required bacula, postgresql & dbconfig packages & successfully created the "bacula" postgresql database.
Thanks again Janis. Your help is much appreciated.