In the name of blogging about hard-to-find things that I finally found:

The MS Word expression for a Hard Return (Carriage Return) is: ^013.
(For those who are going 'Duh!' right now: 13 is the decimal code for a CR in ASCII.)

So, I had a document that used to be HTML and contained a huge amount of space between each paragraph. To fix it I did a search and replace where the search string was "^013^013^013" and the replace string was "^013^013" (one blank line). I kept doing the search and replace until no instances of the search string were found, and – Presto! – all the extra space is gone.

There might be an easier way to do this, but there it is. :)