Here's the letter again, albeit addressed to Michael Gove:
Education Secretary Gove and his staff are clearly unaware of some very well-established results: (1) studies done over the last 100 years show that spelling instruction has very little effect on spelling accuracy; (2) studies done over the last 100 years show that the formal study of grammar does not improve students’ reading and writing; (3) studies done over the last 25 years show that systematic intensive phonics study only helps children do better on tests in which they pronounce lists of words out-loud. It has no significant effect on tests in which children have to understand what they read.
The best way to make sure students “leave primary school with a strong command of both written and spoken English, with high standards of literacy” is to encourage wide self-selected reading, which is only possible when all children have access to reading material.
Professor Emeritus, University of Southern California