The bronze worm nut is pulled up tight into the cover using the hexagon - it locates the mainshaft bearing. The worm is then normally entered from the outside (Rob's must've been screwed right through) and then the dust cover is screwed on, with a felt washer behind it.
I've generally found the worm nut to be pretty tight in the casing and usually, the dust cover will come off on its own.
With what Rob now has (and assuming that he wants to re-use the worm nut), he's going to struggle to hold it steady whilst undoing the dust cover.
The steel cover over the kickstart spring, incidentally, is just a push fit - and almost impossible to remove without marking it.
Would it be possible to find a nut with the correct thread (or any other component on the bike that uses the same thread) and put a slit up the side of the nut and grip it in the vice using the split in the nut to lock it tight?
Done this a few times, fit the bronze part in a large 3 jaw chuck grip it on the large plain part so the chuck jaws are just up agains the steel outer cover. Lock the chuck and undo the outer steel cover with the correct size spanner. Watch for the bronze part slipping in the chuck. I have been successful every time with this but there is always an exception.