I agree that some of the information is based on opinion, but the progress in her ability and maturity are fact i.e her academic progress has been remarkable since her time at her Junior school - should we send copies of her reports or bring these on the day of the appeal?
It depends whether the opinion is yours or that of her teachers. The former will not carry any weight, the latter may well do, depending upon how much firm evidence there is. You should send copies of the reports in advance of the appeal. If I recall correctly, the reports at her school are not particularly lengthy.
My daughter was below the average, below 115.
You have a lot to prove on scores below 115, so you really must focus on the academic case. Extenuating circumstances will be more relevant the closer they were to the test. The fact that you were not able to spend time with her preparing for the test will not impress a panel as mitigating circumstances.
The Head Teacher is fully supporting us in the appeal and has given my daughter his backing with a score of 3, which I believe means he thinks she will succeed but with some reservation.
You need to know what that "reservation" is, because you will certainly be asked why it is a "3", and not higher. What is the second number - for attitude to work?
Her expected Sats results are 5' for Maths and English.
Most appeals will show Level 5s. If you can get the school to provide sub-levels, and they are 5a or 5b, they would help your case. Do you have any other evidence such as reading test results or CAT scores?
and extra curricular activities.
Unless these demonstrate academic ability - e.g. playing chess at a high level, or playing a musical instrument to a high level, they not relevant to the appeal. You may be asked about them if the panel can't think of any further questions but they want to appear polite, but I think your case will be all about academic evidence and why she did not achieve 121.