It seems that I have a talent for breaking things. I’ve been wondering why no-one has commented on here for nearly two months and it turns out it’s because I’d managed to completely remove the comment forms from the blog entries.
I’m not sure how I did it, but I do remember “tampering” a little bit with some of the site templates a little while ago. I’ve refreshed the offending files and it should all be working again now, at least until I find a way to break something else.
Thanks to Sarah for letting me know.