Hi Jane & Francis
Firstly, may I suggest that Forum Admin move this post to the "Appeals" section, where it now belongs, even for Bucks-specific appeals?
Your son is in a good position for an appeal with 2 scores close to the pass mark, plus support from the Head.
I hope that will be compelling enough in itself, along with your appeal letter and other evidence, and you were right not to submit examples of schoolwork with the Appeal submission. They are only expected at the appeal panel itself.
The panel will not expect perfect spelling, grammar and punctuation at this age. There are very few children who achieve that standard at age 11+ (or for some years beyond in many cases!)
I faced the same dilemma last year at my child's appeal. My best suggestion to you (with the benefit of hindsight) would be to produce two or three of the best exercise books from the 3/5 core subjects - (Eng/ Math/ Sci/ Hist/ Geog) and flag one or two very good pieces of work with post-it notes, as the panel will have very little time to look at the books. The "flags" may help, but on the other hand they may encourage the panel to look elsewhere - you can only try to guide them to the best work that your son is capable of.
Photocopied "selected" work is generally not considered as good evidence, and nor is computer printed work, as there is no evidence as to who produced it. (One could argue the same on homework really, but that's a discussion for another day!
I would not try to amplify the evidence with references from sports coaches or others at this stage. You have made your academic case, as has the Head, based on the evidence. Seek to re-state that case briefly and effectively at the appeal panel.
If, by talking to class teachers, you discover any other important evidence that should be submitted, such as a previously unknown exam result or test result, ask for a written statement.
Otherwise, just work out how you will present the evidence you have already submitted to the panel.
Best wishes - and good luck