Well, since no-one has had an answer yet, I thought I'd provide a possible answer to myself. I did find one pdf which seems to be a reasonable introduction to the theory online and appears to be freely available: Kraft and Procesi - Classical invariant theory: a primer (Wayback Machine)
Again - I'm clearly not an expert on this subject and have only started reading up on it, so if someone knows a better reference than this it's appreciated.
