Try to borrow a soft faced hammer (copper), give the heads of the screws a good whack, using a square shaft screwdriver and adjustable spanner (aka shifting spanner), holding the spanner down close to the job, turn the screwdriver clockwise (as if to tighten the screws), then anti-clockwise and see if they undo.
If they don't (and I don't know what you have in the way of tools), try this method. Using a sharp center punch, make a mark with the punch at 90deg (aka right angles) to the screw head, try to make your mark on the screw head at approx 11oclock to the slot in the screw, and not too close to the edge of the screw either, that way you can still use the screwdriver to extract it after it comes loose, make sure you have a hammer (ball pein) of at least a decent weight, not too heavy not too light, after making your initial mark, alter the angle of approach and hit it again in an anti-clockwise motion, this will destroy the screw for further use but they are cheap enough, cheers Dennis
