I tried again on Sunday morning believing it was perhaps North American traffic that was slowing things down. Whether or not this was the case, I ended up with a more reasonable 10-13mb/s download, which meant about 3-4 hours in total to download a 50Gb package. It seems to have done so successfully although I haven't yet installed or tested it as I have the dilemma of having Ortho4XP for California and San Francisco HD installed.
As the Orbx package only covers the top part of California, I'm wondering if there's a way of removing part of the Ortho4XP files for that area, to avoid duplication and to avoid the sim trying to load two lots of scenery. But also to keep the lower half of California until this has been released by Orbx. Anyone have any ideas how to do this?