Thanks
Thank you for the suggestion RGB1 - I cleared out /var/log and /tmp (the wonders of shift+del!) and now the drive has 3.7 GB on it and the server is happy again. There is no way that there were over 175Gb of messages, so I'm guessing somewhere a file was incorrectly allocating space - perhaps following a less-than-orthodox shut-down.
I'll look into crontab to see if I can set up a routine deletion of old logs.