Many thanks for the detailed reply. I think I may have been overly influenced by the glowing recommendation for Libraries in the Calibre user manual!
Reading your reply and thinking about it I have decided that I had better move back to one library and tags. I hadn't realised about Restrictions before you mentioned them. I've now tried them out and I see what you mean about the flexibility it gives - changing from one virtual library to another is just as quick as having separate libraries and is much quicker than using tags on their own. And I noticed how, as you mentioned, the available tags changed with each library.
A few questions if I may:
1) When I set up the Saved Searches I had to leave them in the form such as tags:"=Non-Fiction", which, after saving, appeared as tags:=Non-Fiction in the Restriction box. I noticed that your examples did not have the tags=: bit. I am using Ver 0.8.67 of Calibre.
2) It seemed to me that using the Saved Searches box gave exactly the same results as using the Restrictions box in selecting a virtual library. Am I missing something?
3) I had a quick look in the User Manual and it talked about creating Genres using multiple tags - I must read it in more detail.