Thanks Laurie, I selected centre pivot, but it still wasn't working.

Someone from another forum told me there was a bug in Maya 8 with extruding and that it didnt effect everyone. To fix it, I needed to download the following mel script:

https://www.alias.com/eng/support/ma...PolyExtrude.mel

And replace the file performPolyExtrude.mel, located in C:\Program Files\Alias\Maya8.0\scripts\others

It worked a treat. I can finally continue with my project. Thanks again for you help Laurie.