What Car? Q&A September 2009 - Driving test blunder
A) We put the question to the Driving Standards Agency, and were told that 'legislation doesn't allow for a test decision to be overturned'. Yes, that may end up costing your granddaughter a few more lessons, but they will only benefit her in the long run.
With 13 minor faults recorded, she was close to failing the test regardless of the disputed 'serious' fault, so we wouldn't argue.
If you still feel aggrieved, there's no harm in asking if the DSA would be willing to test your granddaughter free of charge next time.