Thanks for the replies.
I always check the printer nozzle patterns before I print. I also profile my printer (with a ColorMunki), so colour accuracy is usually not a problem.
I use a CIS filled with good quality German manufacture inks (OCP -
http://www.ocp.de/content.php?lang=en).
I can usually produce a run of (say) 6 identical A4 prints, all with the same colour balance from Photoshop.
I've now managed to print to PrimoPDF to produce a PDF file with the 4 images that would make up the A2 print, and they all look pretty similar in colour, so it's probably a printer related problem rather than Qimage. I'll give it another try, checking the nozzle pattern _after_ the print run as well as before.
Jim