Hi Upset Mum
I am sorry you've had such a miserable weekend, and I'm glad you've found this forum.
The first and all important thing to demonstrate at an appeal is very high academic ability. From what you say, your son seems to put in a pretty good performance, and was predicted to pass his 11+, so give it a go at appeal, provided you really feel he will thrive at a GS.
When you see the Head, try to pin him down on his real view - does he feel your son is suitable? On what basis was he predicted to pass his 11+? What was the Head's ranking? (The Heads have to rank all the children in order of the likelihood of them passing the 11+, and if your son was in the top 10 it will be a big help to you, the top 20 is useful, outside of that you may be pushing water uphill.)
What you need to show is that your child has everything it takes to pass the 11+, and the end result was poor because of the loss of his Great Grandmother, in that order.
Start to work with the Head on facts and figures - predicted SATs, past test results - general and VR/NVR, evidence of strong schoolwork, etc. Your case needs to be hard-nosed! Extra-curricular interests have only a very small part to play - the Merit at Guitar Grade 1 is worth a brief mention, as it shows good application to work and practice. Likewise the Progress Prize, but leave the rest out and only mention them if asked at the actual appeal.
As you will know from the bumf that Bucks send out, the chances of a successful appeal on 116 are not brilliant, but if you think your son is right for GS and the Head agrees, give it a go - you have nothing to lose.