I noticed that the gear selection on this UMM was very vague and sloppy. When I looked at the linkage, there were two main reasons. Firstly, the lever that goes into the back of the gearbox (the shorter one), was loose on its shaft. I managed to tighten that up with a 13mm spanner.
Then the bushes at the lower end of the lever were very loose. I dismantled the lever and found that the bushes are plastic (nylon?) and were worn. So I turned up a new set out of brass. These are a perfect fit and have reduced the play in this joint considerably. The gear shift is now much more precise. There is a grease nipple on the housing, so a squirt of grease now and then should keep it in good order. Although, how to get to this once the floor is back in, I don’t know.