Purchased rental movie won't download

Hi

I purchased a rental movie for the first time for my iPad (not the one that went in the pool, the other one!) via my pc, and mid downloading it keeps saying 'error -50' and not completing.

I have tried downloading it direct to my iPad hoping it would say it was already bought and do I want to download it, but no luck. It just makes out that I've never bought it before. Is that because the 48hrs are up even though I never watched it?

Any advice what to do? Or do i accept I've lost my money and start again? :(

Thank you