Ok, after reading someone's post recently saying they need cutting off, and as my panels are already off, I decided to have a go!
The trim is off by removing a couple screws on side of door and one in handle. Note the use of a good pair of pliers holding the rod nice n tight. Use these or risk the rod popping out and breaking a clip (i did this.)
Now get some nice big pliers and firmly twist the knob! Once it's moving, you can slowly pull the knob off pin.
Look at the ridges on top of rod which were what the old knob was bonded around. These are why the knob must be firmly twisted before it is pulled (ouch) There shouldn't be any need for a hack-saw to cut the button (sounds better here) off.
Now the knob is pulled off, attach new knob to ensure fit. These 4 quid ones off ebay are ace as the plastic tapers off, ensuring a nice interferance fit which can easily be removed to take off, and replace door trim.
Now pat yourself on the back and admire your shiney, new knob up-down action! It's a far easier job if you get someoe to hold pliers tightly on rod!