I would attach the letter about the teaching problems, but not write anything about this in your submission. Let the evidence speak for itself. (Pity it mentions maths as well as writing!
Have you also got evidence from the school that he is on target for level 5 in English, and do they highlight recent rapid progress?
If so, I think that should be the thrust of your case.
I would certainly include the CAT scores because the maths and NVR are so good.
At the hearing you should give a rueful, semi-apologetic smile, and say "Typical boy - much better at maths than at English - just look at his CAT scores! - but he has now reached an acceptable standard in English
Then wax lyrical about his passion for maths!
It may or may not work - but it would be the best approach, and offers the best chance.
Don't forget to give reasons for wanting a place:http://www.elevenplusexams.co.uk/appeal ... -school#c2