I can not get Postgres to listen on anything but localhost
I just wanted to make sure that this showed up as solved.

I had unknowingly installed a version 9.5 and a version 10. I had been editing the version 10 config, not knowing that both versions were running.

sudo service postgresql stop
sudo pg_renamecluster 10 main main_pristine
sudo pg_upgradecluster 9.5 main
sudo service postgresql start
sudo pg_dropcluster 9.3 main


Make sure you have copies of your postgresql.conf and pg_hba.conf files before you start as these got returned to default state when I did the upgrade.

Now it makes A LOT of sense why my config changes weren't taking effect ;-)

Many thanks to Colin 't Hart for the invaluable clue!