I have just had a reply from Bookeen about the strange fonts/symbols.
They gave me instructions to delete a couple of files from the Cybook and this restored the correct fonts. I then re-installed my SD card into the Cybook and the same thing happened again. I changed the SD card (for the exact same type of card) and all is now well. The card must be faulty.
Bookeen were very helpful but hadn't seen this problem before.