Good first attempt if you haven't already buggered the heads.Try placing your screwdriver on the screw and tapping on top of the screwdriver gently with a mallet or small hammer. :baffled:
I've done this as well, as a last resort. It does work.If you really bugger the heads up, use a small drill bit to drill the heads off, then when you lift the cover off there will be a stump for you to get a vicegrips onto. Worked loadsa times for me over the years.