Siggsy, I have had a least six broken wheel bolts on my valver, all front, Its is cause they are cheap nasty renault ones made from weak alloy, and they are soft. They stretch when you tighten them a little, and then sieze, so when you go to take them off they just shear at the waist caused by the slight stretch. To stop this put some copaslip on the threads when putting wheel bolts in. To get them off more easily than drilling, whip disc off and use vice grips on threaded section that remains on inside of disc, cause there is no pressure they usually move failry easily. hope that helps. Dont worry about them snapping on car, it is very unlikely to happen, but dont run about with less than the full four bolts on each wheel, although I will admit to doing it myself for a short spell.
Alex M