Lock the metal piece with a pair of vice grips so it can't turn and have someone hold on to it. Wrap a non stretchy rag around the knob and twist up the rag with a screwdriver (sort of create a figure of eight with one bit around the knob and the other with the screwdriver through it). Get it tight, then poor boiling water over it (obviously watch out for yourself). Wait a couple of minutes, retighten the rag and twist.
The soaked rag should grip without damaging and the hot water will allow the plastic to very slightly expand initially and then contract breaking its lock on the rod - hopefully.
Best of luck and don't burn yourself.