How knowledgeable do you have to be in the given subject? I know this is a somewhat obvious question and I have tutored GCSE students before in maths with good results.

This depends on the subject and your students’ expectations. If there is a large gap in ability between yourself and your student, you may be able to get away with tutoring a subject you are not an expert in. For example, I am not particularly amazing at math but I would feel comfortable tutoring a 4th grader in arithmetic. However, I would not feel comfortable teaching a college student calculus. Just be honest with yourself and your students about how well versed you are in a given subject and let them decide whether they feel you are an appropriately educated tutor.