Hi mate, think it just unscrews, mine is missing and there is a threaded bit sticking out, looking for a silver replacement for mine if anyone has one.
You first unscrew the stubby mast and then the bit you are left with has a nut that you just undo. It will then push down into the rear wing. You will needs to unclip the trim the other side of it in the boot and you can then get to the underneath
Tried to do this today and looks bloody impossible!
First I unscrewed the stubby- ok easy did that.
Now I'm faced with a nut but that's on very tight and I don't want to force it. Is that the correct nut? Do I need to get to the other side of it? The other side is virtually impossible to get at