Howdy Ian,
Thanks for including the files - we're getting somewhere now
Opened the .mid file - there are a total of 5 staves - "Jenny Lind" (an empty staff, as Herbert mentioned usually happens), then Staff, Staff-2, Staff-3, and Staff-4.
Opened the .not file - I see that the "Conductor's score" part displays two staves that are braced together. However, when I use Part/Define, I see that the Conductor's score should contain 4 staves.
In the .not file, Staff/Hide resting staves is on
Having that option on while manipulating staves and copy/paste functions will sometimes show you some unexpected results, mostly because of the "out of sight, out of mind" effect. You simply don't see the staves that are really there but Composer does, and has to deal with them as well as the ones you see. This explains the strangely appearing extra staff - it was hidden because it was "resting" (had no notes), and by default any time you add measures, any "resting staves" are revealed, so that you can see exactly what's happened to the score.
I'd strongly suggest (1) have Staff/Hide resting staves off when you're doing such manipulations, because then you'll be able to see exactly what's getting affected in the score and (2) delete any unnecessary staves (two of the staves don't have any notes in them that I can see).
ttfn,
Sherry