Hi Craig,
first of all, I recommend you to do "apt-get -f install".......after that, try "apt-get remove mrtg" and "apt-get dist-upgrade" again.
If you upgrade successfully, you can try install mrtg with "dpkg -i -D mrtg" and if you have problems, check "dpkg --force-help" to force the installation.
Dont forget the good old ./configure --prefix=/opt/mrtg && make && make install if everything goes wrong... and rm -rf /opt/mrtg once you've got a stable solution.
Olivier
