@Mike: See, I tried waiting a day or two and then putting some Red Mist on but it just didn't give a good result for me at all, and my bottle of RM is now consigned firmly to the window cleaning section of my washing tray! I much prefer the Last Touch stuff as a pure QD, which I admit I've never tried using post-wax. I think I'll give that Blackfire stuff a go, I need to order some more clay anyway so I can combine the two.
I suspect a big issue is that I've never owned a black car before, and I wonder if my technique was fine for the blues and reds I've had before, but needs improving on black where every little detail shows.