A post from the Drupal support list requested:

Does anybody know of an existing module which will extract tags from
the body of a post (i.e. a line like "Tags: tag1 tag2 tag3") and pass
them to the taxonomy system to tag that node?

I've had a skim through the Taxonomy related module list on drupal.org
and didn't spot anything...

Cheers,
Dan

Took me longer than I wanted it to take, but at least it's of the minimum quality I find acceptable.

Dan, this one's for you. Download, works for Drupal 5.x