Brian
I had this made by a chap who machines bits for my motorbikes.
It screws onto the broken stud and allows a 3mm bit to start.
I've tried it out and the 3 left hand bit hardly made a mark in the stud.
So now I've ordered some hi quality 3/4/4.5mm bits for hardened/stainless steel and 2 different makes for comparison.
I've not given up just yet but i'm close.