Well another try:
I've managed to drill a 6mm hole 20 mm deep into the back of the hex driving recess using an HSS carbide drill. Went in remarkably easily. Next size up that I have in HSS Carbide is 8mm - total rejection, just polishing the edge of the hole. Tried another brand new HSS carbide in 8 mm with the same result.
I can only assume that the core of the screw has been left relatively soft with the rest at 12.9 hardness.
It seems that 12.9 steel needs to be brought up to 400 Deg C to soften and I'm getting no ware near that with my propane torch. I do have a Mapp gas cylinder but that only burn a few degrees hotter so probably not the solution.
I've a pair of big old Oxy-Acetylene cylinders out side in the safety cage but I dread to think what the state of their hoses is - not been used in a few years. The logistics of getting them near the bench will be challenging.
. . . more cogitation
