Thank you for sharing your question with the Arduino community.
Manage Libraries only shows the libraries which are in the Arduino Library Manager index (http://downloads.arduino.cc/libraries/library_index.json).
You can find your library in the following directory on your computer and in Arduino IDE under Sketch > Include Library:
At some time in the past Manage Libraries did list all installed "custom" libraries in addition to the libraries available from Library Manager index, but that behavior was changed some time ago.
I hope this was helpful!
Yes thanks @nachoherrera, it was. All understood. In fact I'd come to that conclusion about the distinction myself after further folder and file browsing yesterday. Although IMO the original structure makes more sense.
Terry, East Grinstead, UK