Thanks for your suggestion, Elliott - it worked.
And David, although I didn't need more than the 10-column limit this time, I can see why it could be helpful to have the option. (The idea you've linked through has a "Done" status, so not sure any new Likes are registered).
One observation I have is that it took a very long time for report to generate, and with only five custom columns in the report (custom columns were Smart Lists). I originally ran the report with six custom columns, but it timed out on each occasion.