Well done Viva!
You are very lucky that your head will back your appeal. I has to persuade ours to because in this area the heads are very much discouraged to support appeals and I think our head has very strong feelings against grammar schools. But she could see I had good points so she did write a letter in the end. I had letters from the head, class teacher and SENCO as ds has AS.
If your dd is dyslexic you should also get another letter of support from the school SENCO, she could write about how your dd copes with her dyslexia. Also, if you know that extra time will be given for your dd in her SATs, make sure that is documented in time for you to submit it. I'm sure you will know by know that you need evidence for everything, so making it known that your dd will be given extra time in her SATs will carry more weight than just saying that she didn't get it in her 11+.
Again, I did something simelar with ds because he has AS, and we won. Our school is notoriously difficult to get in on appeal as well, ds is one of only 3 children in the whole school that are there on appeal.
I was also advised to give evidence about how children with AS cope with exams and about what sort of procedures are often put in place for them, and I got some great leaflets from the NAS about this because I felt it would help explain why ds passed but didn't score highly enough to guarantee a place. Maybe you condo get something simelar from a dyslexia charity?
Drivinmecrazy, I don't think the panel will access anything that you don't provide, so give copies of her reports yourself. Support from the head is always good, but you need to focus on why your dd needs a place at the school, not why the school would want her as an asset to them.
But my advice is only as a parent who was in this position last year, there are experts on here that will give better advice than I can.