Or is it feeder schools/individual SATS for students that are considered?
It is the SATS for the students in this cohort at this school.
If the latter, how does this affect students who didn't do them
They are looking at the cohort as a whole, not individual students. The question is whether this cohort performed better or worse than previous cohorts.
And does the education system really believe that a child's performance at 10/11 years of age is an accurate predictor of achievement at 16/17/18?
It is a good predictor but it is not perfect. But they aren't using an individual pupil's SATS to predict their grades. They are using the SATS results of the entire cohort to predict whether this particular cohort would be expected to outperform previous cohorts at this school. That then allows them to decide whether it looks like predicted grades are optimistic, pessimistic or about right. Even if the cohort as a whole would not be expected to outperform previous years, an individual student may, through hard work, get ranked higher in the school's prediction resulting in a higher grade.
if there was a huge anomaly between the results of students this year to previous at any particular schools, surely then the evidence collected by staff at that school could have been moderated by those who would normally have marked exams to the exam-board standards to check the accuracy of the predictions fro that specific school
Given the huge variability in the evidence collected, it would have been extremely difficult to moderate predicted grades in this way. I'm not sure it would have been possible.
Well that's changed the goal posts with the all the Scottish exam results being upgraded to expected grades! That should have some effect on the English results don't you think?!
My personal view is that the Scottish grades for this year no longer have any credibility. The jump in the pass rate is too much to be believable.