If those screws are totally knackered or refuse to budge ...with pan head ones I hammer a 1/4" drive bi-hex socket, slightly smaller in diameter of the screw OD onto the offending screw (I think there is an imperial size that works best but can't remember the exact size). This is after grinding a few m.m. off the front face of the socket to remove any chamfered 'lead-in'. The bi-hex 'V's bite into the screw OD plus the shock of hammering the socket on has never failed me yet to successfully remove one. I think the AA/RAC etc. have a similar design of tool for removing customers lock-on wheel nuts, if no key tool is availlabe. On the countersunk ones I drill a hole and hammer a torx type screwdriver bit into the hole...which is slightly larger than the hole.
