Hi Bryan
Thanks for the reply.
It's true that I have the custom design upgrade on another one of my sites (Kevan Pegley), I don't actually have it on the site under discussion (Tanamerah). But thanks for the offer.
I have now removed the tagline in Settings > General, so the site now looks like it used to. It remains a puzzle that the tagline didn't appear until about ten months after the site was created!