When doing my anti lift bushes (I just used TT ones) 4 of the bolts came out fine, one bolt wouldn't come out and the other took about an hour of lubing, undo 1/2 turn, do up 1/4... spray lube... rinse repeat. If you've broken one off you could try buying an Alum bar off of eBay, it dissolves the steel bolt, but not the aluminium. Chances are it's just a few broken threads locking it solid. I know you have broken it, but this will help loosen it up, they are rather tough to simply drill out or use an easy out on. You should be able to see the top of the bolts coming through the sub-frame when looking through the wheel arch, so would be able to apply (and maybe some mild heat) to the end of the bolt and let gravity do it's thing. Don't let it get on the other bolts though, or take them out first!
This vid gives you the rough idea, but it will dissolve a drill bit without touching Aluminium, so it's quite strong stuff :)
Cheers and good luck
Stu