Actually, I've just done something else, that works, much more simply.
I opened the bookmark manager, clicked on 'Imported from IE' to show all of them.Then Ctrl-A, Ctrl-C. Then clicked on 'Other Bookmarks' and did a Ctrl-V.
They all now appear at the higher level, then did Organise/Reorder by title and they are now at the higher level and sorted into order.
Amazing, I didn't think something as simple could work!