[Arran]

 Don't use screwdrivers to pry knobs off, EVER! Sometimes you can unbolt the chassis and move it forward enough to get behind the knob, if you can do that squirt some WD40 or PB Blaster on the shaft and let it soak, this will hopefully break up the rust or whatever is holding the knob on. Then you can use either a rag or a thick string wrapped around the back of the knob to get a better grip. If that does not work you can sometimes wrap the rag or string around shaft enough to force the knob off with pressure.
Regards
Arran