shear off disaster

The manual 300tdi has a big problem....its stripped down to bits for an engine rebuild.However one of the P gasket/water pump bolts chose to shear off a good 6mm inside block while in use.

Its the one that would be the base of the "P" if it were upright and so is close to the block edge and not a lot of metal away from a gallery or bore.

OK,so drilled it out most of the way...then tried to be clever.... always a mistake....reverse thread extractor.....slightest resistance.....snap...I know,spare me the 20/20 hindsight please!

Now I've got a hole mostly filled with very hard tap that will not be drilled.Ive managed to get a flat surface with a flat end punch and now plan to try drilling fine holes around the sides of the tap as I have been advised that the only hope is to shatter it,so if I can get some play either just pull it out or shatter it from the side with a punch.That or take the block to a gurt big engineers and have a chunk milled out.Im going to have to sacrifice the blocks thread and helicoil at the very least anyway.

The placing precludes freezing and sledgehammers.....so....any ideas?Other than dont be a lazy sod in future?

thanks guys

shocks

Did the same thing myself once cleaning out the head bolt threads on a 454 chevy block. After p*ssing around for days trying to drill and break the tap out, had to take the block and get the broken tap spark eroded out. No major machining work, no other damage. I've got no idea who does it round your way, but your local machine shop should know.
As I recall wasn't too expensive either.