ahh the best method is get a good pair of pliers (not longnose)and lay them on their side and grip the outside edge of the screw with the pliers and carefully turn the screw anticlockwise it will crack off then undo it with the cross head driver i always do it this way its a very common thing.other than that use a small sharp chisel and make a cut in the edge of the screw with it and drift it around anticlockwise re new the screw though and put some copped slip on the thread to save any future agro.
adam bike doctor
May 2010