That or else there's a bit of of grit or something on the mating surface, causing run-out.

I usaully just use a brill bit the size of the screw head, drill the head off and use a pair of mole grips to extract the rest of the screw as usually its the presure of it being tightend that makes it hard to get off in the first place so once the pressures off it comes out no problem.

:thumbup:

wd40/penetrating oil and leave for a couple of hours after removing the disc helps too