Thank you, that was good to know. I had tried a few times but decided that there must be some sort of SD lock that had broken. Knowing that someone else had done it successfully made me keep trying.

I kept trying, in the end with cardboard and tape. I knew it was in properly, but the phone still wouldn't see it.

I tried and found that even though the back case was on fully... Turns out the kickstand magnet is missing! I tried holding a magnet in the right place, and now the SD card shows up!

Now I just have to find a tiny magnet to put in place (or a new back cover, but that might be harder to find). I see that in the past another forum member with the same problem used a cut to size magnet from an old pair of earphones. https://talk.maemo.org/showthread.php?t=50463