Sorry for the slow reply but thanks a lot!! You are totally right... I only have the free version. Stupid of me, I didn't realise it had that limitation. Thanks again for answering my question!
It could be a license problem. Since this year's new release of mental ray you have to have a paid license to be able to use batch rendering. Normal rendering with mental ray in Maya will work as normal you just have to buy a license if you want to use batch rendering. So if you want to render out an animation without a paid license you'll have to render out every frame manually within maya.
If you do have a license then I don't really know what is going on but it could be a problem in your render settings.