I have to copy a file to a folder (see below) but the file manager seems unable to access the internal memory of the N810 to do so. Presumably there's a way it can access this memory but I can't find it.
Or do I have to resort to some arcane Linux jiggery pokery?:-(
If so, is there any definitive documentation of how to do this kind of thing?

For Linux OS (and others), you need to copy this file as
"/usr/share/doc/notecase/notecase.key" (alternatively, you can copy it to
the same directory with "notecase" executable).
Make sure to make this file readable to all your user accounts.