I've been unable to shed any light on this problem. I've tried creating a new project set up for prores from the word go, and pasting it in, but no luck. Have also asked a friend who knows Final Cut inside out, and he's clueless. It seems there's no possible reason for it to be doing what it's doing. So it looks like the only option is to start again from scratch a third time. Oh well, at least I'll get some practice out of it. Silver lining and all that.

Now that I'm getting started, I was wondering whether anyone could advise me on the best codec to use. I'm about to start using the aforementioned prores422 (HQ) variant, but if for any reason someone thinks one of the other prores codecs would be more suitable, please speak up now. I don't want to start from scratch a fourth time :p