Well here's the odd thing. The misfire hasn't affected the way the engine runs, and before the diff seized, I had been using her as my daily driver.
It does misfire if I give her some serious beans, but I don't drive like that, so I don't get the misfire. I've also discovered the it runs on 4 from cold faster than it did, almost like whatever the issue was, is fixing itself.
I've even wondered if the fuel had gone off, which has been diluted by a subsequent fill?
I don't know, but I took it round the block after doing the diff, and she drove like a dream, although the front brakes judder, and the clutch is a little softer than I remember it, probably caused by me driving it off the road, with the back wheel dragging.