This photo is of the before state. The ball is dirty and has no grease, the catch is a bit sticky and when you move the stick to the right to select reverse the white plastic bearing moves up towards to top of the gear lever thus reducing actual gear linkage travel. The copper shim reduces that upward movement.. I've annotated the photo.
Steps
1) remove the leather shroud
2) Unscrew gear knob
3) Unbolt the rubber gear seal/cover
You can now manoeuvre the gear change half way out of the tunnel to give you access to the mechanism.
You can now access the big circlip, remove it, you will need stout circlip pliers, I found my cheap pair just didn't have the rigidity necessary.
4) remove big circlip
5) Manoeuvre the lever and plastic bush out of the socket, I found there was a wavy shim steel spring at the bottom of the socket. the plastic bush sits on this I presume to remove free play.
Inspect the plastic bearing for wear etc. If all is good then clean it up and apply some grease to it, maybe lithium grease. I didn't have any and just used bearing grease. If its too badly worn then you need to think about replacement ...
Make a shim, I used copper sheet cos I had some, 1mm thick I think.
Gear change beforeGear lever notesCopper sheet shimReverse steps to reassemble, clean and grease all parts, insert plastic bushes and gear lever back into housing (a faff) place the shim directly on top of the white plastic bush and then replace the big circlip, the circlip will hold he shim in place. Ensure the shim isn't lodged in the recess the circlip fits into, adjust shim as necessary. Hopefully this will help, I found the plastic bearings in mine had no real wear.