Don't know whether you should dare ask what points in particular they liked about you that encouraged them to ask you for a second interview - it could go either way, even if you put it that you are a keen self improver!
Cathy - good luck (belated) for the interview. I shall think good thoughts for you. As for asking why they brought you back in - why not? On most interviews I've had, I've always asked either what they saw on my CV or at first interview that they liked. It's a good intro into expanding on that particular aspect or quality.
