Help. Need to unscrew my aerial base!

Hello people. My mate was playing with my aerial, and the bit that the whip screws into snapped off. So there is a base attached to my roof, a whippy bit, and a metal bit that holds it all together. But they don't go together well anymore. SO. i went to my local dealer and asked what they recommended. They took all my money and gave me a whole new ariel whippy bit and base thing. Fun fun fun.

Get home, and look to attach the new base. Problem, i barely remember what the guy said to do. He said theres a bit to lift, or take off, or something, near the boot flap. I found a plastic cover, but attempting to remove it met with some resisitance. I'm unwilling to force it, particuarly as the car is technically still mums.

Help/advice wanted!