Well - what do you know? I told my hubby about my post on here so he just went ahead and tried it with our wooden chopping board (much braver and stronger than me)! And he's got the jolly thing off!. He slowly turned it forward with a bit of force, then back with a bit of force - and when he came forward again it moved a wee bit. So he just kept turning the handwheel forward as if slowly started to rotate on the shaft and as it got looser started it "winding it out" off the shaft as he went. Incredible! Very definitely not to be tried if you've got weak wrists and they sprain easily like me. If anyone tries this I recommend they get someone whose wrists and arm strength are of the husky, sturdy variety.
Anyway I thought I'd let you know just in case anyone else has this problem (surely they must have and will do) and wants to know about how we got it off. It did, of course, have a couple of hours of "drench it in penetrating oil" and give it some taps treatment first. Kind regards and keep safe... Megan (and my hubby Clark who, Yes, is my Superman!) :-D