how to get the power knob off a Sebo K1 to get the cover off?

Hi, following the very helpful instructions I reckon to open up my Sebo K1 to try to diagnose why it has stopped coming to life - however, the only step in opening it up I can't work out, is how to get the power knob off - without which the casing will not come off as the knob overlaps the casing edges. The knob has power control, so rotates, but won't unscrew and I don't want to force it unless that's the right answer.... Many thanks