An ounce of prevention......
Here is a site with instructions and some pics on how to remove one:
http://alan.mcreynolds.googlepages.com/howtoremovebrokenglowplugs-mercedesom606
Ounce GPs removed ream/clean the Carbon out of the holes.
When you put in the new ones use a Never-Sieze type compound on the threads or if you are real picky and can find the stuff Beru makes grease for the Glow Plugs.