Thanks for your thoughts on the matter folks, I have found something interesting.
When I was attempting to do this download I was at work and using the 'wifi hotspot' function on my phone to connect - now, I've never had a problem with this in terms of functionality before and indeed my KT took the opportunity to stick a couple of updates on my Facebook while it was connected. However, as soon as I got home and connected to my home wifi, it sync'd and downloaded the book instantly with no problems at all. I don't know how big this book was in terms of file size, but I'm wondering if there is a limit somewhere in the 'hotspot' app that means it won't download files over a certain size and the KT was interpreting this failure to download as due to a dearth of disk space?
Anyway, it seems to be working now. Incidentally, has anyone else given up on having an SD card installed because it slows the system down and 1000 books should be enough for anyone?