Glen
Not trying to tell you how to suck eggs, but here is my 2 cents worth and hope its of use.
Get a set of ezyout or stud removes as Steve has said. The ones you wont to use are; once you have drilled the hole you then screw the ezyout into the stud,bolt as its a left hand thread it will (normally) unscrew the broken stud,bolt,as you are screwing it in. Remember these are left hand thread. After 20 odd years in engineering one word of advice when useing ezyouts Slowly Does It, if it get real tight stop drill, the brocken bolt out completely and re-tap the thread. The last thing you want is to be removing a brocken ezyout,(this is not fun at all).
Well hope this helps
Regards
Sarge