Yup, that did it. And, by the way, I noticed that, in my actual project file, the base track was also frozen. I must have clicked that button by mistake at some point as I wandered in a newbie haze around the interface. I unfroze it and I'm much happier with the base line now! So much so that now I need to re-record the vocal and rebuild the demo file. Songs never quite grow up!
Thanks to everybody for their help on this. I've learned quite a bit!