The ink cartridge is full and, as I say, every other document is printing fully black.
I've looked all over Word, and tried Word help (which is notoriously mis-named) but can't seem to find anywhere that I can check if it is in draft form or how to change it if it is. I think you're right, though, in that the problem is specifically with this document, but I'm buggered if I can figure out what the problem is.