My son was sucessful during a HT appeal. He missed NVR by one point but this was always his strongest subject. The school had evidence of such with high NFER scores from year 3/4/5 which 'proved' he underperfomed on the day.
The written piece was also looked at obviously, luckily this also was a strong point of his. His books from year 5 were taken to the appeal.
I think at this stage they genuinely look at the overall picture...our HT certainly gave us this inpression afterwards by saying 'he is a grammar school boy..!'
We were lucky, but this is the madness of a test taken on two days when if they have an off day it can change everything.