As far as I know, you have a good case for appeal, but I'll leave the details to Etienne.
Here's a link to last year appeal leaflet from Herschel website. It is the only link to any appeal advice in Slough at the moment.
http://herschel.slough.digitalbrain.com ... dmissions/
I just wanted to comment on your school choice situation.
If your appeal is successful, you will receive an offer for any Slough grammar listed in your application form. However, there will be a good chance that Langley will be full. They are usually full with straight passes with scores above 115.
There is no point in having Herschel as second choice, because they will also be full with first choices.
Your best bet is to add Slough Grammar as second choice.
You need to go to Langley grammar to add it to the Sough consortium application form, AND also add it to the Common application form for the council.
Please ask if you any question.
Best of luck