You should be able to redownload the SD version at no cost. See:
Thanks for that but i don't think it solves the problem.
Firslty, when he downloads the HD films from the itunes store physically on his Itouch, they download fine it's just downloading them from Itunes. Although when he downloads one from his Itouch, all the other downloaded ones grey out and cannot be watched.
When the Itouch is syncing, an error messages does not come up. All that happens is that the films just don't tranfer to the Ipod. Apple said "When syncing an HD purchase from the iTunes Store, iTunes will automatically sync the SD video for devices that require SD content.", this does does happen but only when the film is being downloaded from the store on the Ipod itself, not on the computer.