I have the exact same model of oven which had the exact same fault!
Gently try & prise the knob forward using a big flat screwdriver behind the plastic -(quite stiff) . Remove & clean the plastic collar behind .Clean off any gunk. Next give the knob-shaft a small dose of Wd40 or penetrating oil . Try & ease the knob back and forward a bit at a time . I gently used a pair of mulgrips and it soon came free.
Halfpintawhelks
October 2008