hi there chris
what i used to do was screw the the allen headed bolt back in a couple of turns then hit that with a rubber mallet. once it was threw enough, take the bolt out and put a big pair of vise grips on the otherside and pull. (not the best way).
but what i recomend is getting a bolt with the same threds and cuttng the head off it, srew it in a couple of turns, and chap it out
hope this is helpfull
Ian.