Hi, I've purchased a few films from itunes (and they are listed as Purchased in my account), but I can't find them on my computer or on my itunes account, and itunes wants me to pay for them again. How do I get around this?

Where did you put them?

Have you failed to maintain a backup copy?

If so, not good, you can redownload some itunes purchases in some countries:

Downloading past purchases from the App Store ... - Support - Apple

If you are in the UK, and the film studio allows re-downloading, and you are signed in with the same account that you bought them with, then they might show under the Purchased link under Quinklinks on the right-hand side of the iTunes store home page for re-downloading - if they don't show then check to see if they are hidden (hiding and unhiding).

But you should be keeping and maintaining your own backup of your downloads and noy rely on being able to re-download any item as they are occasionally removed from the store by the content provider (film studio, tv company, record company etc).