Thanks all.
I think I'll delete everything and start again
No bookmarks or anything - just need to note the page of my current book.
Again, Thank you - a relief there is nothing wrong!
Edit: Looks like there was definately corruption. Plugged the device back in and calibre started to complain about the media.xml file... I tried to delete and recreate but calibre still complained. In the end, I just formated and started from scratch - gave me a chance to clean up some of the book titles and tags so not all bad