Thank you, Stratman. The rest of the nozzle check was unexciting.
I guess the next question for me to think about is whether to spend £80-ish on a head replacement on a printer that is about 6 years old, or replace the whole printer at roughly 2-3x the cost.
Thank you for taking the trouble to respond.
I can see now that I've probably leapt to a wrong assumption in my original post.
Attached is nozzle check, showing split lines for heads in the middle. Also attached is test doc.jpg showing the effect on printed text.
My MX925 printer prints black text correctly for every other scan of the printhead (i.e. in one direction). And for the other direction, the black is split horizontally (or ghosted into two parts of approximately equal weight) separated by about 1/2 mm, resulting in an out of focus appearance...