Your Kobo is activated to a different person than the book is licensed to. This can happen in a number of ways
1.) When you first used the .acsm file you had a different activaiton record.
2.) The transaction within the .acsm file was already used by someone else.
3.) You are trying to reuse someone elses .acsm file (or book itself).
For 1. This most commonly happens when you were originally anonymously activated, and then either authorized with an already used AdobeID, or deactivated and reactivated anonymously again.
For 2. This most commonly happens when the transaction id is resused, as a server side misconfiguration - while this does happen, it is fairly rare.
For both 1 and 2 you will need to work with Kobo to get a new download link issued, to get the book to be licensed to your current account
For 3, I assume this wasn't what happened, but I put it in for completeness sake - This is not allowed.
Thankyou Jim, for your help - i have also had a helpful email from Kobo - so
I'll give it all another try with this knowledge