From student handbook 2015/16, page 69:
When each marker has completed their marking they send the results to the lead examiner who will review
the marks and form an opinion of where the pass mark should be set. This will change each session,
depending on the complexity of the paper, and the students’ overall performance.
This suggests to me that if you sit an exam in a session where students as a whole haven't done that well, perhaps due to not having studied as hard, then the pass mark will be lower & vice versa.
The 'depending on the complexity of the paper' could be done prior to anyone sitting the exam however they don't set an initial pass mark at that stage. So why are they waiting until people have sat it?
Why does the lead examiner need to see how people have performed on the paper before forming an opinion on the pass mark? Is there no academic judgement taking place?