What does Losberne quiz Giles about? What is the "clincher" in his argument?

Losberne ask Mr. Giles and Brittles to go through the events of the night before about six times each, and they start contradicting each other more and more, with each repetition. I didn't see the word "clincher" but Giles gets all confused and the two begin to contradict each other. Finally Giles admits he can't for sure say if Oliver is the boy in question.