Oops sorry I forgot about that. No, I didn’t have them set to “front page”. I’m sure that’s why it’s not working!
Yes the name of the template is confusing. I immediately assumed the front page template had some special markup indicating it was the “home” page, so I was reluctant to use it anywhere else. I know it’s silly, but even knowing it’s OK to use, I still feel hesitant to change all my posts to a template called “front page” – seems wrong somehow!
It would also help if the widgets had a description saying they worked on that template only.
I’ve just realised there’s no way to change the template for my posts other than to edit them all individually. Will it work if I make a copy of singlepost.php in my child theme and paste the code for the ad into it directly?