Fixed it!
Tanks to Mary and Ray for your suggestions, and to Tomaz and Martin for looking into it for me. However, the eventual solution was hit upon by Julian Ridden, the Moodleman himself.
I had a chat with Julian this morning... he was initially as perplexed as I was, but he eventually worked it out. Turns out the News Forum itself had somehow become corrupted. He fixed it by deleting the News Forum completely, then switching to Social Format (which then created a new Social Forum on the page), and then switching back to Topics Format, (which then created a new News Forum). He then deleted the Social Forum that had been created during the switch to the Social Format, leaving only the new News Forum on the course page. Everything went back to normal with the new News Forum now in place.
It turned out to be a simple fix, but it wasn't so obvious as to why it wasn't working (at least not to me)
Thanks Moodleman! I owe you (another) beer.