Thank you for your quick response.
OK, I've tried deauthorising it and then authorised again. ADE says that the device has been successfully authorised to my Adobe ID but I still get the error message on the Kobo.
So I tried re-sending one of the titles from ADE to the Touch, choosing to replace the existing file when prompted to do so.
The book still won't open although my Adobe ID has the correct authorisation on the Account settings of the Touch.
Any other ideas welcomed.