File the top of the broken bolt flat (will stop the drill drifting to one side)
Centre punch as central as possible (in case you end up drilling and tapping later)
Drill through to suit largest stud extractor without damaging threads.
Spray soak in WD40 or similar for 1/2 an hour.
Get a set of Stud Extractors (good quality
shouldn't snap)
Voila...Bobs yer Uncle.
If that doesn't work then drill and tap to suit.
Thats what I would try anyway.....
