as chromatic hints at here, you should make sure you have the latest SVN::Mirror. i know we've seen problems with svk in the past with empty commit messages, and this was (part of)? the solution.
It seems I had two versions of SVN::Mirror floating around. Version 0.72 (the latest) was in /usr/lib/perl5/vendor_perl, but there was an old one in /usr/lib/perl5/site_perl (which I guess was probably being used instead). We'll see how I get on from now on.
Many thanks for everyone's suggestions! Paul
