Did you close that if… thing at the end of your list in the sidebar?
Edit. I did not “create” a functions.php – I just copied it over from a theme that had it.
I did close the “if”, but hadn’t tried copying a working functions.php over. After doing that it worked, and I then stripped out everything but the sidebar registration bit to see if there was an extra function I was missing… and it still worked. That was the state I thought I was in before posting my question, so clearly I wasn’t spotting a typo somewhere.
Thanks for the hint and sorry for the bother,
Mike