Those lower rear overrider screws are described as socket button M6 x 20 in the parts manual. It seems in recent years MMC have been fitting cross head screws though, requiring very short screwdriver, so poor purchase. When I finally got mine off I replaced with stainless socket button screws allowing simple removal with allen key.
I managed to remove mine by jacking the car up (and placing it on axle stands). I could then use an impact driver which required very little force. Now replaced with stainless socket screws.
