I have a recent Cooper and i believe they changed the way the spoiler is attached using a far stronger adhesive and only two bolts meaning that there is a lot less flex in the spoiler. I had an absolute nightmare fitting my white brake light as the spoiler just wouldnt budge. In the end I covered the whole area with white grease, smashed the old redlight to get it out (only way it was coming out!) and then had to squeeze the white one in, again not easy - patience, the right angle of entry and force is what you will need. Good luck!!
Cometh the Hour, Cometh the MINI