If your HT is on side and can provide you with lots of evidence of improvement....your appeal won`t be until May 2012 at the earliest...then go for it.
My DD1 failed on one paper, having scored well in 2. The HT was great, her form tutor also provided her with lots of extra work to do at home too. By the time we had her appeal in the July her ability was a level 6 in maths. Then we just had to produce evidence as to why it was so low in the first place.
She`s now thriving at Grammar school and excelling in maths!!
We put 2 grammars and 2 HS on our CAF and got none of them.....