Hi campbellfrompyrford,
The article below may be able to help you with this issue.
Click on the link below to see more details and screenshots.
I've quoted some helpful highlights for you:
iTunes Store: Where can I download and sync my purchases?
You can download or transfer content you purchase from the iTunes Store to various Apple products. The content you can download may vary depending on which Apple product you use to make your purchase. Access to your content may also vary depending on which Apple product was used to make your purchase.
Purchases made using your computer must be completely downloaded to your computer before you can sync the item to your iPhone, iPad, iPod, or Apple TV.
You may also override the download queue and download your purchases using your computer no matter what device was used to make the purchase. For example, you can purchase a movie on an Apple TV and the movie will be added to the Apple TV download queue. However, you may also go to iTunes on your computer, sign in to the iTunes Store, and choose Store > Check for Available Downloads and be able to download the movie using your computer.
Checking for available downloads from your computer will not allow you to download movie rentals made on Apple TV, iPhone, iPad, or iPod touch. Movie rentals must be downloaded to the place of purchase.
Some content types are not available in some iTunes Store countries.
I hope this information helps ....
Have a great day!
- Judy