Movie won't play on Apple TV

A movie I bought from iTunes on my iMac but it won't play on ATV, just gets locked in "loading" state.

1. I have tried all the tips such as power down, restart, reauthorise/re-authorise, run on wifi rather than LAN etc etc etc

2. the movie has been "locked" i.e. my 48 hours has commenced and I can't run it on iMac as it is tied to another device

3. I can access music off my iMac

4. I selected a preview of another movie (on iTunes on the ATV) and it runs fine

5. It has worked fine previously

but I have to say I haven't used this setup to watch a movie for a while. And after this experience it may well be a while longer...

Help!