Up until now I thought I either had to download and install updates manually, or set up an FTP server locally.
Whilst idly investigating setting up the OSX FTP server I found a much better alternative.
In case someone else is living in the same state of frustrated ignorance I reproduce the instructions here:
fix to allow WordPress automatic plugin installation/update:
A) Change the owner and permissions for the entire WordPress installation.
Assuming you installed WordPress in /Users/.username./Sites:
$ cd /Users/.username./Sites
$ sudo chown -R :_www wordpress
$ sudo chmod -R g+w wordpress
B) Add the following to /Users/
and henceforth you can auto-update your local test environment.