GVK.

Never needed to take the head off, we used to have some lengths of square tool steel we used to whack in the centre of the broken plug and use a reamer handle on, heat up the remaining part of the plug and wind them out.

I had a phase of just undoing them with the 1/2" windy gun, if they snapped they snapped.. No point fannying about..

Although I heard you can use a TX55 TORX BIT to good effect :P