Primary cover damaged screw heads

[Steve.S]

I have read that applying a little course grinding paste to the screw head and tapping the screwdriver into it can help, although I've not tried it myself. You could also try both a Philips or Posidrive to see which fits best.
Otherwise you will have to drill the head off. If you find the drill is going off centre, which it probably will, you can re centre it using a small bit in a Dremel.