That's the beauty of tapping it Clive you don't even have to be in the middle. However, adding to the debate, IF I were to try to drill those out almost to the threads and it was a one shot deal or bust, I would machine up a small plug that would fit in the top of the hole using what thread there was left. This would have a pilot hole in the middle. I would then use a face mill (no, not a drill) in the pilot hole to face up the broken stud so it was flat. Then I'd use a drill, slightly larger to account for wear in the pilot hole, to drill down the middle of the stud. It's a bit of work I know, but last time I did this it was to save a whole front hub where the studs had sheared off. It works. You can use another stud as the pilot guide, but of course you still need a lathe really to put in a perfect centred hole. That's the sort of thing that you can easily get done - on this forum at least. I recall doing this for a bolt for Gary, post it, I'll drill it and post it back.