Thanks for your advice 3d0g.
I have now downloaded the kobo app, downloaded my books from the kobobooks library and synced my kobo. The book is finally ON my kobo which means that if I hadn't wasted the entire day trying to work out how to get there I could read it!
The process took much longer than I expected.
In retrospect I should have deleted everything from my library except the one new book that I needed to transfer. As I don't normally use the Borders or Kobo apps I had forgotten that they like to download and sync EVERYTHING!
Anyway I now have 2 copies of all the books (bar the one difficult one!) I have purchased from Kobo. My Borders books are all still there as are the ones I only have in ADE.
In all a good result.