It seems only fair to report I needed to do another two strips of images today. All 100mm wide and ideally 600mm tall.

I dropped my selected images on some A3+ pages and then moved them around to create the columns. Final positioning was with the Ctl and cursor arrows as suggested above. They worked a treat. Not a single white line between frames. One of my columns was a bit jagged as I must have made a mistake with the CTL Left/Right. But that was no big deal and easily cleaned up with the trimmer.

Thanks again folk.