Thanks for your reply. Having taking in your comment about parts of names not actually existing and having done some more experimenting I now realise that the anything you work with must involve books (in fact, one of the error messages I came across was that no books were chosen). I think I now know how to find my way around Genres.
One final point. I noted the warning in Search and replace about the risk of nuking your library. In fact, whilst experimenting I did manage to force Calibre to close a couple of times - fortunately without any lasting damage. However I've decided to play safe in future and only work with the lowest levels. Despite it being allowable I don't intend to put books in higher levels. I'd say that an important thing, before starting to create your system, is to look very carefully at all the books that will be involved and to think long and hard about the structure. Oh, and avoid any spelling mistakes.
And why persevere with Genres? Quite simple - having a structured tree is vastly superior to having a flat Tags system.