Broken Spark plug

[kawasutra]

I'm not sure of what the inside of a spark plug is like tbh but as you've got one out already, why don't you cut across it and try drilling that in a vice etc first before you try and drill the one out in situ?

You may be able to drill small pilot holes as suggested already but try and do a couple or three in a row and get a screw driver into it and unscrew it that way?

I thought of this idea but do not have a work bench or a vice at hand. I will see if my neighbour is willing to let me try this on his vice.