Using what was suggested that works best for me seems to be this.
1.Select the surface of the box in the 3D view where the offending line occurs and delete that surface.
2.Now mirror and the line is not visible. Problem solved
This has to mean that for any mirrored objects it is far better if they are developed as empty shells, and not solids.
http://www.tomah.com/DElliott/box.gif
Thanks