Have you tried a strap wrench to hold the red bobbin in place while you undo the nut under the slider? It looks like the slider is supposed to spin as it's not locked on the spindle.
And it’s off! My mate used a vibrating drill thing (you can see how it all went wrong initially!), which did the job in seconds. He shredded a rubber glove by holding on to the red side though!
So now that this is fixed it’s my turn to entertain you. One of my pillion footrest rubbers came out and I’ll be b*ggered if I can get it to go back in. I’ve tried warming it in boiling water, washing up liquid, pulling with pliers, hitting with a mallet but it just pops out again. So, suggestions welcome!
