Thanks @EricL for your advice - I will need to use force and some tools to get the knobs off.

Thanks @Dave-1 - I can't see any screw or pin hidden in the underside. I'll need just try forcing the knob off (either by hacksaw or the wrecking bar ...

Thanks for the advice, @Nailbag. I'll try a hacksaw and cross-fingers I hope the other knob comes off too.