ian35mm may well be right.
There's not entirely a level playing field because OoC parents are usually left to do the administering themselves. And it's quite correct that some Bucks primaries mark whereas others don't.
The headteacher manual says
Strict adherence to the instructions issued by the LA in connection with testing is of utmost importance if the fairness of the allocation procedure is to be maintained.
- but it's not entirely clear to me what the instructions are with regard to administering familiarisation. The manual mentions another document -
Please refer to page 11 of the Administration & Marking Manual in the Familiarisation and Practice pack
- Not sure I've ever seen this. I wonder if OoC parents get to see it?
Let's wait and see if anyone can shed any further light. If not, then I think you have every right to ask the school what exactly the instructions are, andyb, and to seek reassurance that your son has not been disadvantaged in any way.
It's much better, as you suggest, to get to the bottom of this now, rather than wait for a possible appeal.