We do not have specific detail of each and every car, and so cannot say what size bolts you have. Given that you are saying you have tried 11mm and 12mm sockets, it may be that you have either locking tabs which are up against the bolt, or they are not standard bolts and will require a special socket. Could you not just pop into to your local garage and ask them if they are special? Also have you sprayed them with WD40 prior to undoing?