The Premium bumper is removed exactly like the standard Mk2 bumper. Can't guess what the 'wingnut' is, but it should become clear to you when you get down to it.
There are lots of discussions and HowTo stuff, but this YouTube video tells it all.
Clicky:
https://www.youtube.com/watch?v=7BUGH4_VMkU
EDIT: If the 'wingnut' is fastened to the wheel arch liner, it sounds a bit like a push-rivet with the ends splayed apart.
TC