"If I knew, I'm sure it wouldn't be there."

There might be just a hint of testiness there; Merlin dislikes the idea of his own mind being messed with as much as anyone.

"It would appear that somehow, our memories have apparently changed places. But seeing as I cannot actually remember what it is for myself at the moment, you'll have to tell me exactly what you know."

Oh, he knows it's something to do with talking to Fujimaru in Uruk. But what about? He hasn't a clue.