you need a small dia hss drill bit that will drill easily into the remaining diameter with a mm to spare all round, then a stud extractor, like > http://www.screwfix.com/p/screw-extractor-5-piece-set/18643 < this has a counter thread, so as it bites in it starts to wind out the remainder of the bolt.
you need a small dia hss drill bit that will drill easily into the remaining diameter with a mm to spare all round, then a stud extractor, like > http://www.screwfix.com/p/screw-extractor-5-piece-set/18643 < this has a counter thread, so as it bites in it starts to wind out the remainder of the bolt.