So, I've tried as you suggested - hitting Shift on a reboot to boot into NOOBS. If I then press Ctrl-Alt-F2, rather than get a login prompt, I simply get a flashing white cursor in the top-left corner of the screen on a black background. I've googled around to try to work out what I'm doing wrong, t...

Great! Thanks Gordon.

Do I need to run any specific commands to mount the newly-inserted SD cards in order to make them accessible?

Cheers!

Thanks Gordon - sounds like the clincher is the bit where you say "I'm just not sure whether you are going to be able to access the CSD with the software available in NOOBS..." - it's the 75KB executable 'sdtool-rpi' from bertold.org that performs the CSD edit so, without that, I guess I'm in the "y...

I would like to be able to use this tool: http://www.bertold.org/sdtool/ on some SD cards to render them read-only. To be clear, the SD cards aren't the same SD card that I use to run my Pi - they are SD cards that I use for other stuff but I'd like to be confident that nothing is able to be written...