Complex sentence: Norman attempted to warn London that he was in captivity by not giving the Germans the second part of his security check, which they did not know about, but was frustrated when London sent a curt reply telling him to correct the omission.
Split into simpler sentences: Norman attempted to warn London that he was in captivity by not giving the Germans the second part of his security check, which they did not know about. However, Norman was frustrated when London sent a curt reply telling him to correct the omission.