OK well it's amazing what you can find if you keep looking.
I found this page here ->
http://doc.m0n0.ch/handbook/faq-hiddenopts.html which describes the hidden options for interface settings in the config.xml file. I then looked up a man page online for fxp (the intel card I'm using) and it says I can use a media setting of "full-duplex" and a mediatype setting of "10baseT/UTP". I assume I place them in the config.xml file as follows?
<media>full-duplex</media>
<mediaopt>10baseT/UTP</mediaopt>
I don't have access to the server right now so I'll test later.