You are right, that was one of 7 special channels (BBC RB 1-6 used to be on the same transponder) added to cover simultaneous games during the big events.
They were indeed all on 12421H, I still have them in my channel list but they are not active at the moment.
I would like to see a permanent upgrade to HD for BBC RB though I only use BBC RB to watch snooker as the normal schedule is mostly the same.
