I just tried transferring the file, via DropBox, from my PC to my Imac and it downloaded and played fine.
I'm not too sure what's happening when I send the file over to a client. I've been using WeTransfer previously for other animations and it's been no problem.
Your extension is absolutely fantastic by the way, please don't think that this issue has any effect on how I continue to use Animator. It's probably just one of those things but I had to bring it to your attention just in case you had a solution.
One thing I would like to ask is about the sizing or dimension of the viewport before I start processing, is there a rule of thumb for optimal size, i.e 1920 or 1200 etc...
Thanks again.