Do you think there is any mileage in the HT/DHT letter repeating the issues regarding the school or should they concentrate on academic evidence/support only?
I think it depends how much evidence there is that progress in English has been affected.
4a could be quite respectable for the end of Y5, but - if Y5 was the main problem with five different teachers for Literacy - one would want to compare 4a with the levels being achieved before
Y5 to draw conclusions.
Ideally one would expect two sub-levels' progress per year.
If it can be shown that English has been held back, then the extenuating circumstances ought to be brought to the panel's attention - and this is best done by the school.