Just wanted to add a follow-up with a happy ending, and to give credit where it's due!
I wrote a letter to Bookeen in Paris telling them about my download problem, and pointing out that I had emailed three times and received no response.
Today I received a reply by email apologising for technical issues, and inviting me to choose another title to replace the one I'd paid for, plus an extra title for the inconvenience.
All is forgiven.
