What is the name of the strange symbol that looks like a toppled "S" or an incomplete infinity symbol and how do I enter one, please?
I think you're describing the sign for a turn.
It's not native to NWC, but you can find it as the lower case "u" in the NWC Extra Ornaments3 font. You can download that from the Scriptorium, instal it as a font, and then set it up as a User Font in Page Setup. You just enter it as text. It won't yield playback, so you could put the spelled out notation on a hidden playback staff if you like, written as in the example at http://en.wikipedia.org/wiki/Ornament_(music) (http://en.wikipedia.org/wiki/Ornament_(music))
Third edit:
I guess Rich and I were replying at the same time, we even found the same Wikipedia entry! I didn't think to check Lawrie's fonts but they are excellent.
I think you are referring to an ornament called a "turn".
If you want this in your music, you have to use a text font to enter the turn symbol and then either enter notes exactly as the turn should be played in your piece or enter the notes as played on a hidden stave while having the note with a turn above it on a visible muted stave.
Have a look at this page http://en.wikipedia.org/wiki/Ornament_(music) (http://en.wikipedia.org/wiki/Ornament_(music)) to see how a turn might be played.
You will find the turn symbol in Lawrie's series of "ding" fonts available from the link in the Scriptorium. http://nwc-scriptorium.org/helpful.html#Fonts (http://nwc-scriptorium.org/helpful.html#Fonts)
Yup, a turn is what it is. Thanks, all.