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