+1 Toadmum - my understanding of the appeal process is this: to be able to appeal to a school, you must first be "rejected" by that school, having applied for a place, and THEN (for a selective school) provide evidence of academic ability (plus any extenuating circumstances) and still you may not get offered a place.
The first part - "you must first be rejected by that school, having applied for a place", requires you to have listed them on your CAF by the deadline set by the county in which you live (usually end October) and then get "rejected" by them on National Offer Day in March (as your child has not met the academic standard) at which point you can appeal....
The only problem with this "understanding" is that when we had a Glos. Council Education Rep. visit our primary school a couple of years ago to discuss Secondary School options, she told us that if our DC had not qualified for a Grammar School NOT to put it on CAF, as we would be wasting a place on the form.
I ignored her and put the GS DS wanted to go to first on the CAF. The Council allocation email informed us he could not have a place because "he had not reached qualifying score" and only the 96 who had qualified had been offered places. We asked for reconsideration - this was turned down because "he hadn't qualified", but his choice of GS offered him a place anyway, without appeal. I am so glad I did not heed the advice given, as I am sure putting it on CAF helped him get a place. Knowing GS was undersubscribed also gave us hope and inspired us to start our appeal process.
It is too late now for 2016 entrance as CAFs have been submitted, but it is advice to keep in mind for future CAF submissions.