Hi Lou,
I got the images to import with no issues, but it did create 13000 tiles. Not a huge problem in itself, but a lot (most) of those tiles are going to be empty in this case, as your mosaic is a long, straight diagonal run. If you could import that in sections - say, 100m at a time - it would be quicker. You wouldn't save much disk space as transparent tiles don't use much.
Your mosaic is big - almost a mile long at high res. The problem with zoom is something we need to fix - the image doesn't show at all at lower zoom levels, and we should change that so that it shows an outline or something. I had no trouble seeing the image once I zoomed in to the correct location, and overlaying it on the map. Choose "overlay" and make sure the opacity is set high.
Let me know if you are still having issues.
Matt