Meg I know I'm not really best for advice but have you tried reloading the image plane inside the inside the camera (representative by a little rectangle). It happened to me a lot in the previous project. If you start bringing it between scenes Maya can loose track of where that thirds plane is coming from. I hope your problems get sorted soon!!
Thanks Shan, unfortunately I removed the image planes before I imported the camera into the scene, so it may be a case of me just remaking the camera if I can't wrestle it into submission (really don't want to, I've already set up most of its animation). Thanks, though!
Ah man, there may be a way of copying the information in the graph editor and assigning it to a new camera to save you loosing all that information. But you will have to make sure its in the exact same position as the camera before. Of course I don't know for sure but it's worth having a look into at this stage of the project.Or you could always just create another thirds plane? :)