Thanks both, this is really informative and helpful. Out of interest, leftysauce, how can you see that J8C0 on RJ and J6C3 on CX aren't bookable? Is this through ExpertFlyer?

I found "availability" on these flights for the first time a few days ago on Amex Travel (I'd previously only managed Matrix and sporadic Priceline availability), but running into the same booking failure.