Sorry to hear of your disappointing news - we went through an appeal early this year (2009 11+) and were successful on 113
and 120 (January born). I received excellent advice on this website from etienne and sally-anne and also others who had been through the appeal process.
You can win your appeal. Focus on academic evidence as much as you can. We requested the paper be remarked which as expected remained 120 and received a copy of the paper which showed which questions were answered wrongly and also the raw score. From this we put together statistics which showed which 'type' of question' was incorrect. Interestingly, despite maths being dd's weakest subject she was highly successful on those types of questions which helped us as our weakest part of the appeal was that she was only 4a in maths sats predictions.
We also had had an educational pschologist visit and conduct tests which gave her a very high iq. This was well worth the money (quite expensive) as the EP made some very nice comments about her ability as well. Also letters of support from the headteacher and class teacher came in handy and taking along carefully chosen school books (which should include English, Maths and Science)
We had some weak extenuating circumstances the panel seemed most interested in academic evidence.
Send your form back with a brief outline of your reasons why your child should have passed and mention this will be followed up with a more detailed analysis (plus EP report if you go down that road). This will buy you time to prepare an extremely strong appeal.
It is quite a journey but be resilient, gather compelling academic evidence and there is an extremely strong chance you will be successful.
good luck x