Helen, as your suggestion seemed the least technical and the epub does have the same problem, I tried your suggestion. It took me a while to find the Tweak function until I tried just pressing T on the keyboard (silly me)

. When I exploded the epub it contained lots of "pickle" files which windows couldn't open. So I forced windows to open them with notepad. But there was hardly anything in there, just something that looked like a Euro sign and I think there was an equals sign. As I didn't know what to do, I left them alone.
I googled pickle file extension but didn't find much, just some stuff about python, whatever that is. I'm really not very good at the technical aspects of computing.
How do I embed a regular font? (Simple replies please!)