not enough time to read all replies before my laptop battery dies

had similar problem with broken brake caliper bolt on my motorbike

very carefully drilled out bolt stub with various sizes of drills until could just see the threads showing on the side my drill hole was closest too, drilling dead center very tricky

then used a dremel type tool with tiny diamond bits to take out even more material until threads could be seen all round

then after soaking in kero over night, used a sharp point to break out the remaining threads, if they wouldn't come a little more dremel etc etc

best bit was that the diamond bits were bought at the $1 shop!