Split a nut with the glow plug`s thread (12x1,25mm).Hold the two halves with grip pliers on the plug threaded part. Put a wrench socket or someting suitable between the cyl .head end the grip pliers for steady support.Let somebody carefully unscrew the damn plug....The plug will break @45Nm,be careful. Of course,any machine shop would fabricate the "hardware"needed. The method has been successfully tested on Beru glow plug on om605(250D euro)engine.Saddly,the pictorial is not active for the present.
Good luck!
P.S.something like this:
http://www.mytoolstore.com/otc/6005.html
Good luck!