Check out
http://southafrica.rockschool.co.uk/guitar which is the local version of the site, it shows the different exam centers. Looks like there are 4 in Gauteng.
That looks like the right book, Ray, but what I'd recommend is that you check out the parent site
http://www.rockschool.co.uk and download their breakdown of the syllabus. It explains what is expected at each Grade and also has some sample music pieces.
I found it easiest to just drive to Marshall armed with the above information and leaf through the books to get an idea of the complexity. Alternatively, there are some tutors locally who are affiliated with the Rock School syllabus and should be able to advise you. You can find out through Music Connection.
