Should I only revise subject knowledge directly relevant to the mark scheme?

While it is certainly important to familiarise yourself with the mark scheme and all the relevant materials, I am very much of the opinion that all knowledge is useful knowledge. Make sure your answers make full use of the information you know will be useful and appreciated by your marker but what will really make your paper stand out is the ability to supplement your knowledge with information from other sources (documentaries, internet articles etc.). This is not to say you have to study the subject with the intensity of a university student, you have plenty of subjects to revise after all, but by consulting alternative sources, even only doing so occasionally, you might find new perspectives on the subjects you've been studying or you may even stumble upon brand new information altogether.