Number plate lights stuck

Hi, what tool do I get the plastic cover off? I've tried a flat head screw driver, a pen and fingernail but I've broken a bit of the plastic on both of the lights trying to get it off, can anyone explain? Really nooby of me but I don't want to do any more damage.
 
They are a nightmare.. such poor quality. I'm pretty sure the only way to get them out is to prise them out as carefully as possible