If you are unable to remove a broken stud or bolt by welding a nut to the stub or drilling it out, or if you have broken an extractor or "easy-out" FORGET IT.
The only way of removing the drill, tap or extractor is by Spark eroding.
Find a local firm with a spark eroder or google for spark eroding mobile service.
This is NOT cheap ( about £75 to £100 plus carriage if in their workshop but several hundred pounds if they have to travel), but may be better than having a commercial vehicle off the road for several days.
see www brokentap.blogspot.com