at a pinch , drill a nice size hole and use an old torx bit from a cheap screwdriver set ,, grind a bit of a lead on it first then tap it in , and usually the actual tap will loosen the bolt enough to turn the broken bit out with a socket on the torx bit. ... dont go daft beating it in though ,,, just a '' nice '' fit is enough to sort it out , if not its time for the real thing.