Hi Martin
Thanks for the image. Place the bracket in a vice with the upper end (larger section of collar) not obstructed by the vice. Gently tap the lip of the plastic collar with a flat ended screwdriver and hammer in an upward direction and the plastic collar should pop out. Alternatively, you could try tapping the lip of the plastic collar from the upper section of the collar towards the opening in the bracket and it might slide out. However, in the image the lip of the collar looks a tad chunkier than normal.