Intermittent stiff gearbox - SOLVED

Increasing difficulty in selecting into the 5th / reverse plane, and gear lever generally stiff, not springing back into the neutral position from this plane.
Found to be that the shift inner cable-rod for side-to-side movement is exposed at the gearbox end when in the 1st/2nd plane. This (and lack of use during COVID) meant that surface rust on the cable-rod restricted movement to & from the 5th / reverse plane as it could not smoothly slide back into the cable sheath.
To fix, i removed the battery and battery tray to access the linkage visible on top of the gearbox (forward-backward gearlever movement is the cable below-rear of the 'box), selected 1st gear to maximise exposure of the cable-rod and lightly abraided the rust off followed by application of grease and several movements into / out of 5th gear to free it up and attempt to grease the cable sheath inner.
To prevent reoccurrence, i now leave the car in 5th / reverse gear when parked so that the cable-rod is not exposed.
Hope this helps someone.