Using Calibre, clicked on the entry. in the right hand pane formats: was empty. Using Windows I looked in the directory shown under "Path:" in the right hand pane. The directory was empty. I did this to make sure that Calibre wasn't simply misreporting the fomat. Since there was nothing there, I used the Calibre recycle icon which would I thought remove the entry (I think I did try the Preferences/Advanced/Check Database Integrity first).
I can't see any way that I could have selected the other books, they were not contiguous in the view I had, and whilst I admit my memory is not what it was, I think I would have remembered holding the CTRL key down for 115 clicks.
To clarify things more, I am using the Calibre interface for everything except when things appear to go wrong when I use the normal Widows tools to investigate, not to fix.
For the sake of even further clarity I am using Windows 7 32-bit, fully up to date, 4 GB RAM, and the data is in a RAID mirrored set. 35 years in IT means that while I am no less likely to cock things up than anyone else, I would normally expect to realise that I had done something stupid
Thanks for all the responses, I hope that this is not coming across as negative because I don't intend it to be, it's just that Calibre is so close to perfect for me that I want to be confident in it, and after I have got over the initial hurdles would like to get involved in the development.