Solved

As Charles hinted, the problem had to do with the "do not check spelling and grammar" setting. It was UNSELECTED, which implies it should be checking. Nonetheless, I changed the language from English (UK) to English (Canada), closed the dialogue and it was checking spelling! Then I went back into the dialogue and noticed the checkbox was SELECTED -- this didn't make any sense... I switched back to English (UK), Selected and unselected the dialogue box, and did all this a few more times and it settled back into a correctly working system. So I can only partially explain what happened there, unfortunately; but it's working for me, fortunately.