Well, I did finally manage to get SP1 RC to uninstall by disabling my virus protection, firewall and Winpatrol (as I had done when I was having the installation problems).
Had to do a registry fix for yet another watermark.......grrr. LOL
Strangely, a whole bunch of security updates were immediately offered. These were partly the ones that should have been offered last update Tuesday, but weren't.
I guess I'm out of luck for any more RC candidates, so will just have to be patient and wait for the final RTM. I hope that they have it available as a full download as well as pushed to WU when that happens.