Guess you talkin about the plug for the ignition amplifier. If ther eis a lead and a plug, sounds like you hav a remote ignition amplifier. Mine is attached to the body of the dizzy and then a two pin (female) plug connects onto that.
Will have to have a look, I remember seeing something somewhere about this. Are you sure you have the right Dizzy. I also seem to remember something about there being different amounts of advance depending on dizzy and model year.
If it is the case you only need to change the ignition amplifier, I know Intermota do a replacement for the DLM8 dizzy, there is a shop in Dunstable that can get them.
Try doing a search either for Lucas DLM8, or Ignition Amplifier, pretty sure you'll find the answer. If I manage to remember anything will post here.