Thanks to Mike Ormond for letting me know that the ringtones are stored in the following directory:

My Device\Windows\Rings

Bizarrely enough I can't "see" the \Windows directory from my PC when the mobile device is connected. Initially I used the "File Explorer" application on my device to move my custom ringtone to the \Windows\Rings directory.

I subsequently found an even easier way to implement my custom ringtone as follows:

• Simply copy the .wmv file to the device (in any directory) - I put it into "My Documents" by dragging and dropping from my PC.
• Select the .wmv file from the "File Explorer" application and select "Set as Ringtone" from the menu

Hurrah - no more nasty factory shipped ringtones.

Incidentally these instructions worked for my M3100 and theoretically should work for any other Windows Mobile 5 device though I haven't tried other devices.