I've just figured out what happened, because I've just done it again, but realised in time to correct it:
When I was doing the real entry (either creating or editing) I must have gone to edit the old entry, and then done the later entry, which overwrote the old entry because it was the last entry accessed.
I know this, as I say, because I've just done it again. My entry from a moment ago contains "richardwinskill.co.uk". I knew I'd already used it in an earlier entry, so when I came to the point where I wanted to enter it, I went to edit for the old one and copied the html. I then switched back to the window with the new entry in and pasted it, and finished the entry. When I published it, the entry wasn't there.
I searched, and found it had replaced the older entry. Fortunately, I could go to "Work offline" and use the cached edit page to restore the correct content.
So, mystery solved. Just a shame I can't recover the other entry in the same way; too long ago…