Thank you for responding.
I have re-downloaded and tried a couple of my epubs with your fonts and CSS. That seemed to go OK. So I have now updated to my fonts and CSS. So far so good. I will see what happens tomorrow with more extended use. Now I'm really confused, but keeping my fingers crossed.
After the testing earlier today I had a further complication when my reader suddenly became unreadable by the PC and I had to reformat it all. I wondered whether the hacking had caused the problem. Now I'm wondering whether it was the other way round and the internal memory was already in trouble when I started to test your hack. Only time will tell.