I have already googled the message, and I have plenty of disk space available on the SVN server (it's about 4% usage of 150 GB).
I have noticed that when I try
echo $TMPDIR at the command prompt on the SVN server I get nothing.
What is making this a little confusing is that I only get this message from one location when I do an
svn diff (that I've tested so far) - this error is not coming up when I try from three other computers (one of which is testing against the exact same repository, the other two are different repositories on the same svn server).
About the only difference I can see is that the broken working copy is connecting to the server by an IP address where all the others are using a server name (although this resolves over DNS to the same IP Address).
I'm hoping that I don't have to scratch the broken working copy and checkout a new one - unfortunately, this is a legacy project and not all changes have been properly revisioned.