Yep, the sewing machine repair man has been!
Apparently my thread wasn't strung in the right way.
At my machine (Janome 6600) there is
a big white (sort of) button;
You can turn it to adjust the tension of the upper
The bit sticking out.
You have to put the thread around it when you string a (new) thread.
(I've tried but I can't make an arrow, nice and red to point out exactly what you need to be looking at.)
Can you see the small thing the thread is taking upwards?
It's made of metal and just a bit thicker that the thread.
When you string your thread you have to check whether this bit goes up as well. This means your thread can get tension.
And that's probably where I went wrong.
Now I know.
And hopefully, you do too.