OK. So I fixed it. I had one last go and the solution is such a basic hack I can't believe it worked. But it did.
For some reason it looks like there's a conflict set up by iTunes between the HD and SD download.
First. Sign out of everything you require an Apple ID for. iTunes, iCloud, the lot.
go to the iTunes Store.
find the movie that's stuck.
toggle between SD and HD Versions. You should see a purchase option and then see a circle with a stop icon (square) inside - the download icon.
try and re purchase the movie and it will ask you to input Apple ID etc. Don't. Cancel.
Toggle to the download icon. Press it repeatedly.
if it doesn't ask you for your Apple ID go back and try and buy it again. Then cancel again. Repeat previous step.
You need itunes to ask for your Apple ID after pressing the download icon.
If you get this request input your ID and password and the error goes away.
SImple.
FFS.