After 2 weeks of intense head scratching, I think I've got my gearbox sorted.
I don't know what caused the initial lock-up, but it seems when I pulled it apart, a pin fell out of the shaft & I put it back into where I thought was the only place where it could possibly go.
Apparently it wasn't the right place for it, so I took it out this morning & put the box back together & it's working properly again... but I've got a spare pin.
I've got the mower back together and I'm tempted to give it a trial on an overgrown lawn this week without oil or sealant to make sure it's fixed. Then pull it apart again & try to find a home for the lonely pin & seal it up properly. I probably shouldn't, but I'm sick of looking at the thing on my workbench