All worked well. Thanks all for the tips.
The reason for the blocking was an white plastic ring that sits inside the nutch at the end of the mirror. The outer diameter of the ring is larger than the inner diameter of the grey plastic that sticks in the base. So it's blocking and I had to pull very strong to remove it. But it worked and now I have the new round mirrors on the car and one grey plastic has a new split line. But now without the white plastic ring. I think the screw secures the mirror enough.
Cheers Oliver
