Thanks for your help folks. I have fixed my problem - probably took a long way round, but it worked!
I ended up deleting all my custom meta data fields then saving my entire library to disk. Then I created a completely new empty library (with no custom fields) and imported all my books into that.
Once they were all imported I could then add my custom fields and all works fine.
That done I would like to learn more about Embed and Polish. I presume thse are both Calibre plug-ins as I cannot find either of these within Calibre itself? Where can I find these two?
In Calibre when I click on "Get New Plugins" an error message box pops up saying "Unable to reach the plugin index page" and when I click on details all it shows is:
https://plugins.calibre-ebook.com/plugins.json.bz2
(is that normal or have I got another problem?)
In my browser I've tried going to the URL
https://plugins.calibre-ebook.com which does take me ao a page with Calibre plugins but I can see neither of the above listed there.
Thanks again for the help and advice.