A few years ago I thought it would be a good idea to change the gearbox coupling as the bushes do tend to wear causing the gearshift accuracy to suffer. I installed an aftermarket part which seemed to be ok quality. However the bushes used for this coupling were made from rubber. As there is often some gearbox oil kicking around in this area some gets onto the coupling.
 
I went for a drive in the SC with my friend Steve. We only got a couple of miles when it was impossible to select second and fourth gear. We managed to get the SC back to my garage and diagnosed the problem. The rubber bush in the coupler has disintegrated as can be seen in the picture below.
 
 
 
I think the excess gearbox oil broke down the rubber bushing over time. The coupler is easily accessible by lifting the carpet between the seats and removing the four screws holding the access cover down. Mark the coupler position and orientation. The coupler can be removed by undoing the clamp bolt and the worm screw as can be seen on the picture below.
 
 

You can either buy a complete new coupler or the suppliers offer separate polyurethane versions of different hardness depending upon your requirements. Below are racing black versions which have little play. They should not suffer from any oil degradation.
 
 
 
You have to drive out the centre pin of the coupler to remove the old bushes. Then insert the new ones and drive the pin back in. Replace the coupling in the same position and orientation then torque up the worm screw and clamp bolt.