Complex sentence: He was asked to send a coded message to London that the Italians had laid the keels for three destroyers, but got the code wrong and said 300 destroyers; shortly after this he was sent "on indefinite leave".
Split into simpler sentences: He was asked to send a coded message to London that the Italians had laid the keels for three destroyers, but got the code wrong; the message said "300 destroyers". Shortly after this he was sent "on indefinite leave".