Data repair and upgrade after update
I'm not sure I'm currently qualified to give advice given how badly I seemed to break my install! I think the problem was the classic one of following outdated advice and relying on Google search too much.
I started with a fresh install following the video and found Composer to in fact be very good, though I think it would be helpful to be a bit clearer about it's function. I really had no idea what it was even supposed to be doing for quite a while.
I think I'm back on track now anyway, lesson learned that maybe I should keep more abreast of changes (just need to find the time!).