Hi Leon,
Very strange problem indeed. I can't help wondering whether it is not an artifact generated by the software rather than the printer. The pattern being printed seems too regular to be a hardware problem. Have you updated the printer driver recently? Has the photo been processed in Photoshop or etc.? How does the photo look on the camera screen?

Been ages since we last communicated. Possibly because its been ages since I've had a good night with the scope to report. Hope the skies around your neck of the woods have been better than we've been getting in Melbourrne over the past year. I haven't had a good night with the scope since July 2013!

Regards,
Robert