someone the other day said something about putting it into 4th and then pulling on the knob. My max-power Punto Mk1 manual tells me that you need to saw the gnob off.
Mine came off in my hand when I did a fast change 1st to 2nd a month or so ago, and for a while kept comping off until I got used to it being a little loose. So just pull at it and it will be free!