Chris York and Seb Stegmann gave the Quins a flying start with tries, but James crossed twice in the first-half and back row youngster Luke Hamilton got a third to give the Blues a 24-24 interval advantage.
There were three more tries for the home side after the break, including two more from James, as they sailed into a big lead. The returning Ben Blair added all six conversions and added a penalty to seemingly make the game safe.
But the visitors ended with a flourish as two late tries brought them to within five points, but the Blues youngsters managed to hold on for a fine victory.
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