Never mind, I should learn to RTM - although the page documenting it was a bit confusing.
Here's what I did just so it goes in the archives for anyone else wanting to do this:
Scott Pettit said:
I have a m0n0wall setup on the end of a fiber circuit, with the WAN interface plugged directly into
the media converter.
The WAN interface needs to be forced to 10Mbit full duplex or else I end up getting a very high
number of collisions.
After searching the archives I found I could use exec.php and force the mode, but how can I edit the
CD such that it will change to this mode when the machine is booted?
I setup m0n0wall over the weekend, came back today to find there was 200,000 odd collisions which
was making the fibre perform very slowly.
Any help would be much appreciated.