the Headmaster of my daughter's current school has volunteered to attend the appeal with me. Do you think this is sensible or counter productive?
Some local authorities would discourage heads from doing this. However, it sounds as if there is no such policy where you are, and even if there were, it's not your problem. If the head has volunteered to support you in person, I would seize the opportunity!
my daughter required 105 in each of two tests, she obtained 115 and 104. I asked for her papers to be re-marked, which they did and e-mailed me with scores of 115 and 105, only to be told four days later that they had a mistake and the original scores of 115 and 104 were correct. This has been very upsetting (again) for her. Is it reasonable to include this in any appeal?
I think you could mention it briefly. Keep it very low key, and very reasonable, and it might elicit some sympathy!
With a gap of just one mark, and such obvious headteacher support, you should have a strong case anyway.