When teachers mark do they look at the examiner report or just the mark scheme ???

The mark scheme - and they are interpreted pretty strictly (but part of the skill of good marking is being able to interpret the mark scheme properly). The examiner's report obviously comes out once the exam is completely marked and an assessment of the entire cohort's performance can be made.

What about more subjective subjects like History or Psychology?I noticed in the mark scheme it says (for both of the aforementioned subjects) "Credit other credible answers" (that were not bullet pointed in the MS). Kind of confused.

Exactly - which is why mark schemes are of limited use to amateurs. Subjective subjects necessarily have to have a degree of interpretation, but you need to be an experienced teacher and marker to know when an answer is worthy of credit because it's essentially fulfilled the mark scheme and when it's just missed the mark. This can be a very nice distinction sometimes.

Mark schemes for quantitative subjects such as the sciences and maths, and to a lesser extent things like economics can be interpreted by an amateur relatively accurately. For subjects like History and English, it's much, much harder.