No, I've taken the gearbox out, rebuilt it with the silicone grease on the balls and thrust bearing.
Put it back together to find the same problem. The gearbox is now out again.
After re-installing the gearbox and finding out it still slipped, I took it out.
With the gearbox out of the car as above, I locked the 2 out drives but could turn the spur gear easily.
The spur is turning the shaft, it's not loose in any way.