Thanks for the replies.
Now I have to worry that I've irreparably wrecked the printer, because it's been like this for months.
I've owned printers since the mid 90s and, while I've never been more than a casual user, this is really the first I have ever heard of this.
The cartridges are transparent so I can see there's no ink in them. They're the originals that came with the printer about eight months ago.
I've changed the black cartridge three times since, and I can see the ink in that one.
Printing a test pattern just shows the black.
Sometimes, when I...