My DS is doing the Tiffin boys test on Tuesday but my friend's son is doing it a few days later for medical reasons. I have heard the same test is used both times. Is that true because if so, surely the late sitters are at an advantage. Surely and urban myth?

Most places use the same test (don't know about your particular one), many until next summer. Foolish of people to pass on information, though & potentially put their child at a disadvantage.