why does my iTunes download keep restarting

Hi, I'm trying to download a movie to my iPad from iCloud that I have already purchased - it downloads the full file (I assume, looking at the file size), but right at the end the download just starts again!! So I don't have a file to watch and the download is just endlessly downloading. Is this a known issue? I've never had it before, I guess I usually just stream though. Any ideas?

I'm on latest iOS and it's an iPad 3.

Thanks!

Matt