Thanks guys, the problem disappeared and I didn't want to go mucking around and bring it back if there was no longer a problem. We'll keep an eye on the calibration monthly.
We have the same problem (more often than not). But as it only seems to happen on trim marks outside the image area I don't worry about it. What actually causes it I do not know, and neither do any of the so called experts. It doesn't seem to be an alignment problem, as the line up tests and the actual image area print as normal.
