My specific question is whether it is relevant mention that DS spent Yr5 being taught in class with 20 Yr4 children (school has several mixed aged classes due 40 children in each year group - classes determined by age - DS has mid-july birthday). I felt this held him back, particularly in the development of his vocabulary. I have a letter I wrote to the school about it last May. Is this a valid argument - or would I be undermining our assertions that despite the lack of challenge in Yr5, DS is predicted to get 5bs in SATS at end of Yr6?
You've taken on board the point that, despite year 5 problems, your son is expected to get 5bs. I don't think it will undermine the KS2 predictions - it just doesn't help the argument that he's been disadvantaged by his English.
You've probably nothing to lose by expressing a bit of concern about maturity (July birthday) and vocabulary.
If you had some optional SATs tests in English from year 5, which were not quite so good, that could certainly support your case.
School does not do CATS or Reading ages so additional academic evidence is somewhat sparce.
The lack of any alternative evidence of reasoning ability could be a problem.
Gifted and talented for Maths and PE
You could try arguing that, although he's a good all rounder, he's particularly strong on the Maths side. (There's a bit of Maths in the VR, but not enough from his point of view! If Bucks had a NVR paper as well, it would probably have suited your son. Try suggesting this - but be careful not to appear critical or to attack the system!)
School reports are good but written in a bland style (think they are all written that way)
I'm afraid a lot of them are!
Music teacher offered to write in support - achieved highest score teacher had ever known in music exam. (not sure if that will help)
I think so. There's said to be a link between music, NVR and Maths. Might help the argument above.
Vicar has written in support explaining how DS showed outstanding abilities to understand and articulate the complex spiritual concepts covered during his Confirmation classes. (again not sure if this helps)
When in Yr5 DS was first child in whole school to achieve highest level in mental maths challenge (weekly tests at increasingly difficult levels) - is that relevant?
Hope that helps a bit.