Firstly, there's the academic side. I think you should refer to all the academic evidence you can assemble to prove that - under normal circumstances - she could reasonably have been expected to score highly enough to secure a place. The wider the range of academic evidence, the better.
(Even though the new Code makes no mention of extenuating circumstances, it seems to me perfectly reasonable to explain the background to the dip in performance - but don't spend too much time on this!)
Secondly, there's the possibility that the school is, or will be, oversubscribed for year 7. You therefore need to come up with reasons for wanting a place at this particular school.http://www.elevenplusexams.co.uk/appeal ... -school#c2
It would be worth taking the time to read right through the Q&As - there may be answers there to questions you haven't even thought of! http://www.elevenplusexams.co.uk/appeals/
Hope this helps.