Thanks, PeterT, that worked a treat! Have made real progress, but have now got stuck at the line
su: Authentication failure
a couple of lines before BACKING UP THE VOX
I used the only password I know, and that is the one I use for all of the other password request stages. Is there a second password, which I have forgotten about, perhaps created at the install of Lubuntu in the first place? If so, is it possible to retrieve it somehow? Once done, at what point can I pick up the task, or do I have to start again from scratch?