I seem only to be able to save programs to internal memory, even with a working, formatted 1 MB memory card installed. Is there a way around this, please?
You can not save to internal memory when a memory card is inserted.
It is a bit hard to see if you are using internal memory or a memory card, but in the file manager, you should be able to see more available space with the memory card inserted.
You are able to confirm that you have saved to the memory card by removing it from the NC100 and seeing that your file is no longer present. When the memory card is reinserted, the file is present again.
If you turn on timestamps in the file selector menu, it'll show L or C depending on where the file actually is. Files are written automatically to a card if it's inserted, or get transferred there if you open a local file while the card is present.