Hi,
I have found a way around it - have to un-check the alpha channel setting in the render settings.
But here is a screen shot of what I meant:
http://i44.tinypic.com/23tr0ap.jpg
On the left is with alpha off, and on the right, on.
It's no problem to just turn off alpha while I am previewing my post effects..
Unless I can preview them directly in my viewport render? thus meaning I don't have to constantly check on and off 'render all frames / current frame' when checking the animation / post effect.
Thanks for your response