Ah I see, that makes sense. I did notice the Nulls that are created in Cinema 4D yesterday - I just hadn't connected them to the Sketchup files. They must be created when you create a 'component' in Sketchup, but Sketchup doesn't visually show a marker for components, you can just tell that they are there from the boundary on the object when you select it. They are pretty similar to Groups in Cinema 4D.
Thanks for helping out with that. I spent about 30 minutes trying to remove them from a stadium file and then decided to give up! Glad I didn't carry on now!