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I recall a case in Kent where a parent won an appeal in similar circumstances (although maths was the only problem result, and his aggregate score met the minimum requirement).
Admittedly, he had some excellent alternative evidence for maths.
He set out to prove at appeal that the 11+ maths result was a blip. He had the results of a series of CATs tests, taken over an extended period, where quantitative reasoning came out at the 99th, 98th, 98th and 95th percentile. Even allowing for confidence intervals, the sheer consistency of these excellent results made for a compelling academic case.
There was also very strong support from the school. “X recently sat a year 6 maths SATs paper and achieved a 5B. X is consistently good at tests. ….. X is in our top set for maths.