DJJ Something simple to check ( before taking the box to pieces) is the camplate plunger. This is inside the cylindrical housing with a large locknut sticking down under the box. The usual setting is with a couple of threads showing, but it is worth putting a clean catch pot under the housing, slackening the locknut and unscrewing the housing completely.
Prepare for a deluge of oil, inside the housing is a hollow pointed plunger and coil spring, and of course the plunger should move freely in the housing. The edge of the camplate can be seen through the vacant hole. Too tightly adjusted binds the camplate movement and affects the ease of gear changing, too loose makes the change less positive, as the plunger has a location function with the notched edge of the camplate as well as helping to rotate the camplate to select the gears. Refill the gearbox by removing the inspection plate and level plug on the gearbox outer cover.
Swarfy