I am disappointed that they are making you go to appeal. If you made a formal complaint to the Director of Childrens Services and they rejected it, I would have recommended referring the case to the LGO. However, provided the appeal panel has been properly trained, this should be straightforward.
The basis for your appeal is that they have withdrawn a place that was offered. They are only allowed to do this in very limited circumstances as set out in paragraph 1.50 of the School Admissions Code.
The clerk to the appeal will have a word with you before the appeal explaining what will happen. You will then go into the appeal and be introduced to the members of the panel and the LA's representative.
The LA's representative will present the LA's case. If you haven't already received their written submission you should receive it shortly. Their case should closely follow the written submission and shouldn't contain any significant new information. They shouldn't be trying to ambush you (although I don't see how they can in this case as it is so straightforward).
Once the LA's representative has finished presenting their case, you will be given a chance to ask questions. The panel should also ask some questions. You can use this to highlight any weaknesses in the LA's case.
You will then be asked to present your case, following which the LA's representative and the panel can ask you questions. That will then complete the hearing. In general you won't hear the panel's decision until a few days after the appeal.
If other appeals for this school are being held on the same day, there is a chance they will be treated as a grouped appeal. If that happens, all the parents will be present while the LA present their case and all will be able to question the LA's representative. The other parents will not be present when you present your case.
Yours is an open and shut case based on the information you have posted previously. Take copies of the offer letter and the letter withdrawing the place (if there was one) with you if you haven't submitted them as part of your case. There will be 3-5 people on the panel plus the clerk and the LA's representative. Make sure you have enough copies to go round.
Without knowing what Surrey have said in their submission to the appeal I can't really tell you what questions you should be asking them. In presenting your case, I would just lay out the facts - you were offered a place and 5 days later the offer was withdrawn. Mention paragraph 1.50 of the School Admissions Code which makes it clear that there are only very limited circumstances in which an offer can be withdrawn. Mention the LGO's decision in case 99C01876 - a head teacher offered places to 5 children incorrectly. The LA withdrew the offer 3 days later. The LGO decided that the offers could not be withdrawn after such a delay.
Personally I wouldn't bother with any other arguments about why this is the right school for your son. You want the panel to focus on the fact that an offer was made and withdrawn so keep it simple.
Provided the appeal panel has been properly trained, they will know that the LA is not allowed to withdraw offers once they are made. This should therefore be straightforward. However, I have come across some badly trained panels. If you lose, you should appeal to the LGO.
Good luck and feel free to contact me if I can help in any way.