Since snapping an exhaust stud on the 4th January, I finally got round to drilling it out.
Looked at the threads covering the subject on here, loads of good advice by the way, and had a go at repairing it with the engine in place.
Managed to drill out the old stud, but fucked up the tapping of the head so am going to drill it oversize and helicoil it back to M6.
Or rather, get a professional to do it, because if I fuck this up I'll have to get the head welded.